This is the fourth part of our learn docker by example series. This blog is all about dockerfile tutorial for beginners. This blog will understand the dockerfile and why we need to create one. So let’s get started.
What is Dockerfile?
A dockerfile is a text document that contains a set of instructions to create a docker image. Docker builds images by reading the instructions from a Dockerfile.
With the help of dockerfile, docker images get created, and docker containers get created with the help of docker images. I have tried to explain to same with the below image.
Now we know what dockerfile is. Let’s proceed further and understand why we need to write a dockerfile.
Why a dockerfile is required?
There are so many docker images readily available in dockerhub, so why anyone needs to create a dockerfile? The most straightforward answer to this question is to create a dockerfile if existing docker images do not fulfill your project requirement, which happens most of the time.
And it might be possible that the dockerfile someone else has written has contained some venerability, so the recommendation is to write your dockerfile instead of relying on someone else dockerfile.
General syntex of dockerfile
The general syntax of dockerfile follows the below pattern
# Comment INSTRUCTION arguments
Lines that begin with a hash, or pound, symbol (#) are comments.
INSTRUCTION represents the keyword in the dockerfile. Instructions are not case-sensitive, but the convention is to make instructions all uppercase to improve visibility.
The first non-comment instruction must be a FROM instruction which specifies the base image to use. Dockerfile instructions are executed into a new container using this base image. The execution order of instructions in the order of their appearance in the Dockerfile.
The instruction is not case-sensitive. However, the convention is to use UPPERCASE to distinguish keywords from arguments more easily.
There are so many keywords available in the dockerfile. Good to have a basic idea about all the keywords before writing a dockerfile.
What is an Instruction in dockerfile?
Before writing a dockerfile, it is crucial to understand what makes a dockerfile. Let’s try to understand keywords with the help of the below example:
FROM ubuntu:18.04 COPY . /app RUN make /app CMD python /app/app.py
In the above dockerfile, the keywords FROM, COPY, RUN, and CMD are the instructions we pass while creating a dockerfile. These instructions perform specific tasks based on the order in which it is written, and these instructions are called keywords in the dockerfile. Some of the important instructions are:
Docker Instructions:
- FROM
- ADD
- COPY
- ENV
- EXPOSE
- LABEL
- STOPSIGNAL
- USER
- VOLUME
- WORKDIR
It is essential to have a basic understanding of the above keywords before writing a dockerfile. Let’s see a quick explanation of each keyword.
FROM
Every dockerfile should start with FROM keyword. In the FROM keyword, we specify the base image to be used. It is always recommended to use a small base image. Follow this blog to learn more about which base image to use while creating a dockerfile.
The general syntax for FROM keyword is
FROM <base-image-name>:tag
ADD
The ADD instruction used to copy a directory(one or more) from the source and add it to the filesystem of the image at the path <dest>. We can also specify wildcards to copy entire files from a directory.
To copy all files from the test directory to /var in the docker container.
ADD hom* /mydir/
COPY
The COPY instruction copies files or directories and adds them to the container filesystem at the path. Similar to ADD command, we can specify multiple source path files.
Syntex
COPY [--chown=<user>:<group>] <src>... <dest> COPY [--chown=<user>:<group>] ["<src>",... "<dest>"]
Difference between COPY and ADD
Most beginners are always confused between COPY And ADD. Since both ADD and COPY are functionally similar, but generally, COPY is preferred. That’s because it’s more transparent than ADD.
The COPY command supports the basic copying of local files into the container, while ADD has some features (like local-only tar extraction and remote URL support).
ENV
With the help of the ENV Instruction, you can set up the environment variables. You can use ENV to define the environment variable which will be available in the container.
ENV can also be used to set commonly used version numbers so that version bumps are easier to maintain in the dockerfile.
Consider the below dockerfile where the env variable is used to set up admin_user
FROM alpine ENV ADMIN_USER="mark" RUN echo $ADMIN_USER > ./mark RUN unset ADMIN_USER
EXPOSE
The expose instruction exposes the ports on which a container listens for connections. The expose command is similar to the -p flag, which we have seen earlier.
We can not expose any random ports; ports should be exposed based on the application we are deploying. i.e., for Nginx, we should expose port 80, and for MongoDB, we should expose port 2017.
Format and usage:
FROM ubuntu RUN apt-get update RUN apt-get install -y nginx ENTRYPOINT [“/usr/sbin/nginx”,”-g”,”daemon off;” EXPOSE 80
LABEL
You can add labels in the dockerfile to manage projects, add license information, or any other reason. You can add more than one label in the dockerfile. The following example shows the use of label keywords.
LABEL com.example.version="0.0.1-beta" LABEL vendor1="ACME Incorporated" LABEL vendor2=ZENITH\ Incorporated LABEL com.example.release-date="2015-02-12" LABEL com.example.version.is-production=""
USER
Use user instructions to change a non-root user. It is always good not to run any services with the root user.
We can also use RUN or CMD instructions along with theUSER command.
VOLUME
VOLUME Instruction is used to expose any database storage, configuration storage, or files/folders created by your docker container. It is always encouraged to use VOLUME for any mutable user-serviceable parts of your image.
WORKDIR
You can set up a working directory by using WORKDIR instructions. For reliability and clarity, always use the absolute path for your WORKDIR. The following example shows the use of WORKDIR.
WORKDIR /go/src/project/ #Absolute path
Now we have a pretty good idea about what various instructions are in the dockerfile. Let’s proceed further and build a docker image using dockerfile
Docker file example
In this session, we will be deploying a docker container using a dockerfile.
Dockerfile alpine example
In this example, we will be creating a docker container using an alpine image, and then we will print the content of /etc/passwd file.
create a folder name dockerfile_example1, and inside that create a blank file and name it dockerfile
mkdir dockerfile_example1 cd dockerfile_example1 touch dockerfile
Paste the below code into the dockerfile
FROM alpine MAINTAINER naiveskill ([email protected]) ENTRYPOINT ["/bin/cat"] CMD ["/etc/passwd"]
The explanation for the above dockerfile:
- From instruction is used to specify the base image which we have used. In this case, we have used alpine as a base image.
- Maintainer instruction is used to set up the image maintainer name. It’s free to text, and you can type anything in the maintainer.
- The entrypoint command is used to define the default app you wish to run when the container starts. In this case, we wish to run the cat command
- With the CMD command, we can specify the list of parameters to the entrypoint command. Here we wish to read the content of /etc/passwd file.
Now go inside the folder where the dockerfile is present and type the below command to build the docker image.
docker build -t my-dockerfile1 . [+] Building 0.7s (5/5) FINISHED => [internal] load build definition from Dockerfile 0.1s => => transferring dockerfile: 149B 0.0s => [internal] load .dockerignore 0.0s => => transferring context: 2B 0.0s => [internal] load metadata for docker.io/library/nginx:latest 0.0s => [1/1] FROM docker.io/library/nginx 0.3s => exporting to image 0.0s => => exporting layers 0.0s => => writing image sha256:ff7a403e93223e6820e919f0e2576921b4957aa8b3237831f9412027ffaf7abd 0.0s => => naming to docker.io/library/my-dockerfile1 0.0s
-t flag is used to tag the image. In the above example, we have tagged the image with the my-dockerfile1 name.
. at the end of the command signifies that dockerfile is present in the current path.
Now let’s verify if the image gets successfully created by typing the below command.
docker images REPOSITORY TAG IMAGE ID CREATED SIZE my-dockerfile1 latest b2e435c3294c 13 days ago 5.6MB
Build the image and run a container as follows:
docker run -it my-dockerfile1 root:x:0:0:root:/root:/bin/ash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin adm:x:3:4:adm:/var/adm:/sbin/nologin lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin sync:x:5:0:sync:/sbin:/bin/sync shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt mail:x:8:12:mail:/var/mail:/sbin/nologin news:x:9:13:news:/usr/lib/news:/sbin/nologin uucp:x:10:14:uucp:/var/spool/uucppublic:/sbin/nologin operator:x:11:0:operator:/root:/sbin/nologin man:x:13:15:man:/usr/man:/sbin/nologin postmaster:x:14:12:postmaster:/var/mail:/sbin/nologin cron:x:16:16:cron:/var/spool/cron:/sbin/nologin ftp:x:21:21::/var/lib/ftp:/sbin/nologin sshd:x:22:22:sshd:/dev/null:/sbin/nologin at:x:25:25:at:/var/spool/cron/atjobs:/sbin/nologin squid:x:31:31:Squid:/var/cache/squid:/sbin/nologin xfs:x:33:33:X Font Server:/etc/X11/fs:/sbin/nologin games:x:35:35:games:/usr/games:/sbin/nologin cyrus:x:85:12::/usr/cyrus:/sbin/nologin vpopmail:x:89:89::/var/vpopmail:/sbin/nologin ntp:x:123:123:NTP:/var/empty:/sbin/nologin smmsp:x:209:209:smmsp:/var/spool/mqueue:/sbin/nologin guest:x:405:100:guest:/dev/null:/sbin/nologin nobody:x:65534:65534:nobody:/:/sbin/nologin
Dockerfile httpd example
This example will use centos as a base image and deploy the httpd service in that container.
Now, create a folder name dockerfile_example2, and inside that, create a blank file and name it dockerfile
mkdir dockerfile_example2 cd dockerfile_example2 touch dockerfile
Paste the below code into the dockerfile,
FROM centos RUN yum install httpd -y COPY index.html /var/www/html/ CMD ["/usr/sbin/httpd","-D", "FOREGROUND"] EXPOSE 80
create another file called index.html and paste the below content in that file.
<!DOCTYPE html> <html> <body> <h1>Dockerfile tutorial</h1> <p>Naiveskill</p> </body> </html>
Verify if the dockerfile_example2 folder contains the below files:
ls -ltr total 16 -rw-r--r-- 1 XXX staff 122 Jun 29 23:06 dockerfile -rw-r--r-- 1 XXX staff 95 Jun 29 23:10 index.html
An explanation for the above dockerfile:
- From instruction is used to specify the base image which you have used. In this case, we have used a centos base image.
- With RUN instruction we are installing the httpd web service into the docker container.
- With COPY instruction we are copying the index.html file from local to /var/www/html/ path in the container
- With the CMD command, we are starting the httpd web service in the background
- EXPOSE instruction is used to expose port 80 of the container.
Now let’s go inside the dockerfile_example2 folder and run the below command to build the docker image.
docker build -t my-dockerfile2 . [+] Building 11.3s (8/8) FINISHED => [internal] load build definition from Dockerfile 0.0s => => transferring dockerfile: 36B 0.0s => [internal] load .dockerignore 0.0s => => transferring context: 2B 0.0s => [internal] load metadata for docker.io/library/centos:latest 11.0s => [internal] load build context 0.0s => => transferring context: 31B 0.0s => [1/3] FROM docker.io/library/centos@sha256:5528e8b1b1719d34604c87e11dcd1c0a20bedf46e83b5632cdeac91b8c04efc1 0.0s => CACHED [2/3] RUN yum install httpd -y 0.0s => CACHED [3/3] COPY index.html /var/www/html/ 0.0s => exporting to image 0.0s => => exporting layers 0.0s => => writing image sha256:3fad6b6f2f7ab0a5f04d42a574dfbf20d0712fb9ea44fd7521c6adeb97c2b173 0.0s => => naming to docker.io/library/my-dockerfile2 0.0s
-t flag is used to tag the image. In the above example, we have tagged the image with the my-dockerfile2 name.
. at the end of the command signifies that dockerfile is present in the current path.
Now let’s verify if the image gets successfully created by typing the below command
docker images REPOSITORY TAG IMAGE ID CREATED SIZE my-dockerfile2 latest a4ca21f02823 3 minutes ago 252MB
Now, Run the docker container by typing the below command:
docker run -d -p 8081:80 my-dockerfile2 c63f5db298dd6656037892c520b6798cb260024c5944d3ea140e338f6e699b5e
-d flag is used to run the container in the background.
-p flag is used to forward the request from container port 80 to localhost port 8081.
type the docker ps command to verify if the container is running:
docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES c63f5db298dd my-dockerfile2 "/usr/sbin/httpd -D …" 4 minutes ago Up 4 minutes 0.0.0.0:8081->80/tcp, :::8081->80/tcp funny_babbage
Now go to localhost:8081 to access the httpd web UI.
If you are getting the below message in the web console, it means your container is working as expected.
Now you have a pretty good idea about how to write a dockerfile. Let’s proceed further and understand the best practice to write a dockerfile
Dockerfile Best practice
Writing the docker file is not enough; the recommendation is to write the dockerfile with best practices to improve image quality. There are a few important steps that you should keep in mind while writing a dockerfile:
Minimize the no of layers in the docker
While writing a dockerfile, make sure to minimize the no of steps in the dockerfile or club one or more steps together to reduce the no of layers in the image. You can read my blog about the docker layers to understand better how layers affect your image quality.
So instead of writing multiple ENV commands in dockerfile:
FROM alpine:3.4 ENV MY_NAME="John Doe" ENV MY_DOG=Rex\ The\ Dog ENV MY_CAT=fluffy
club similar commands together.
FROM alpine:3.4
ENV MY_NAME="John Doe" MY_DOG=Rex\ The\ Dog \
MY_CAT=fluffyChoose a docker base image with utmost care
Every dockerfile starts with a base image, and the recommendation is to use the lightweight base image for your dockerfile. You can read this blog to get a clear idea about which base image to use while creating a dockerfile.
Use a .dockerignore file
If you have unnecessary files/folders in your directory which are not needed by your build. It is always good to have a .dockerignore file that works similarly to .gitignor that will exclude the files from the docker build. In .gitignor, you can specify the list of folders and files that should be ignored in the build context.
Keep the instruction on top of the dockerfile which are less likely to change
Docker will build the container from the cache if the steps are not changed. The moment it finds some change in the dockerfile, it will start building the image from that point, ignoring the cached layer. So it is highly recommended to keep the instruction on top of the dockerfile, which is less likely to change, which will ultimately improve your container build time.
Wrap up
Finally, we have come to an end to this lengthy tutorial. We have started with the dockerfile definition and understand the various components of the dockerfile. We also built and deployed some docker containers using our custom dockerfile, and finally, we understood some best practices for writing a dockerfile.
I hope you like this tutorial. Please do let me know if you found this tutorial useful.
happy learning :)
A useful tutorial for me and it helps to practice dockerfile, thank you very much.
Here is one tip for dockerfile_example2, the base image might be better specified as centOS 7, like ansible/centos7-ansible. Because CentOS team doesn’t support official mirrors for centOS 8 since 2022.
For practicing dockerfile ,directly using centos7 would be quicker, but I find it not bad to address the error caused by latest centos base image