Mastering Dockerfile Syntax: A Comprehensive Guide to Boost Your Containerization

  WHAT IS DOCKERFILE:      A Dockerfile is a script or configuration file used in Docker, a platform for developing, shipping, and running a...

 

WHAT IS DOCKERFILE:

    A Dockerfile is a script or configuration file used in Docker, a platform for developing, shipping, and running applications in containers. Containers allow developers to package an application and its dependencies together, providing consistency and reproducibility across different environments.
   The Dockerfile contains a set of instructions that Docker follows to automatically build a Docker image. An image is a lightweight, standalone, and executable package that includes everything needed to run a piece of software, including the code, runtime, libraries, and system tools.

The syntax for writing a Dockerfile and Format

1. FROM
    A FROM statement defines which image to download and start from. It must be the first command in your Dockerfile. A Dockerfile can have multiple FROM statements which means the Dockerfile produces more than one image.
Example:
Syntax: FROM <baseimagename>[:<releasetag>]
FROM java:17
FROM nginx:latest

2. MAINTAINER
    This statement is a kind of documentation, that defines the author who is creating this Dockerfile or who should you contact if it has bugs.
SYNTAX: MAINTAINER <authour’s details>
MAINTAINER Firstname Lastname <example@gmail.com>

Example:

MAINTAINER Manu Manjunatha <techiev@gmail.com>

3. RUN
    The RUN statement defines running a command through the shell, waiting for it to finish, and saving the result. It tells what process will be running inside the container at the run time.

Example:

RUN unzip install.zip /opt/install

RUN echo hello

4. ADD and COPY

    If we define to add some files, an ADD statement is used. It basically gives instructions to copy new files, directories, or remote file URLs and then adds them to the filesystem of the image.

To sum up it can add local files, contents of tar archives as well as URLs

Example:

Local Files: ADD run.sh /run.sh

Tar Archives: ADD project.tar.gz /install/

URLs: ADD https://project.example-gfg.com/downloads/1.0/testingproject.rpm/test



5. ENV
    ENV statement sets the environment variables both during the build and when running the result. It can be used in the Dockerfile and any scripts it calls. It can be used in the Dockerfile as well as any scripts that the Dockerfile calls. These are also persistent with the container and can be referred to at any moment.

Example:

ENV_URL_POST=production.example-techiev.com

${ENV_URL_POST}

6. ENTRYPOINT

It specifies the starting of the expression to use when starting your container. Simply ENTRYPOINT specifies the start of the command to run. If your container acts as a command-line program, you can use ENTRYPOINT.

Example:

ENTRYPOINT [" sh start.sh"]

7. CMD
    CMD specifies the whole command to run. We can say CMD is the default argument passed into the ENTRYPOINT. The main purpose of the CMD command is to launch the software required in a container.
Example:

CMD ["program-foreground"]

CMD ["executable", "program1", "program2"]
CMD [ "npm" "run" "start"]
Note: If you have both ENVIRONMENT and CMD, they are combined together.

8. EXPOSE

    EXPOSE statement maps a port into the container. The ports can be TCP or UDP but by default, it is TCP.

Example:

EXPOSE 3030

9. VOLUME
    The VOLUME statement defines shared volumes or ephemeral volumes depending upon whether you have one or two arguments.

Example:

1. If you have two arguments, it maps a host path into a container path.
           VOLUME ["/host/path" "/container/path/"]

2. If you have one arguments, it creates a volume that can be inherited by the later containers.

          VOLUME ["/shared-data"]

10. WORKDIR   
      WORKDIR sets the directory that the container starts in. Its main purpose is to set the working directory for all future Dockerfile commands.

Example:

WORKDIR /directory-name

11. USER
    It sets which user’s container will run as. This can be useful if you have shared network directories involved that assume a fixed username or a fixed user number.

Example:

USER techiev

USER 4000

12. ARG 
    A variable that can be provided at build time is defined by an ARG Instruction. Once it has been specified in the Dockerfile, you can specify it using the –build-arg switch when creating the image. The Dockerfile supports multiple ARG instructions. The only instruction in the Dockerfile that can come before the FROM instruction is ARG.
After the image is created, ARG values are not accessible. An ARG variable value won’t be accessible to a running container.

Example

ARG image_name=latest

FROM centos:$image_name

docker build -t <image-name>:<tag> --build-arg image_name=centos8 


Here's a basic structure of a Dockerfile:

# Use an official base image

FROM some-base-image:tag

# Set the working directory inside the container

WORKDIR /app

# Copy application files into the container

COPY . /app

# Install dependencies or run other commands

RUN apt-get update && apt-get install -y some-package

# Expose a port (if needed)

EXPOSE 8080

# Define environment variables

ENV APP_ENV production

# Specify the default command to run on the container start

CMD ["app-start-command"]

Once you have a Dockerfile, you can use the docker build command to build a Docker image based on that file. The resulting image can then be run in a container using the docker run command.

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