(Day 12) Task : Dockerfile Components & different commands :-

What is a Dockerfile?

• A Dockerfile is a simple text file with a set of instructions to build a Docker image.

• It defines what goes inside the Docker image — like OS, libraries, dependencies, app code, environment variables, etc.

Components of a Dockerfile :-

Here are the most commonly used instructions:

1. FROM :

• Purpose: Specifies the base image you want to use.

• Example:

    FROM ubuntu
  

• Notes: Every Dockerfile must start with a FROM instruction.

2. MAINTAINER :

• Purpose: Specifies the author , Owner or maintainer of the image.

• Example:

    MAINTAINER ="yourname@example.com"
  

3. RUN :

• Purpose: Executes commands inside the container during image building.

• Example:

    RUN echo "Aditya Sharma" >/tmp/testfile
  

• Notes:

  • Each RUN creates a new image layer.

  • Best to combine commands with && to reduce layers.

4. CMD :

• Purpose: Executes Commands but during Container Creation.

• Example:

    CMD ["echo", "Hello World"]
  

• Notes:

  • Only the last CMD in the Dockerfile is used.

  • Overridable at runtime (docker run).

5. ENTRYPOINT :

• Purpose: Similar to CMD, but fixed — can't easily be overridden this has higher priority over CMD , first commands will be executed by entry point only.

• Example:

    ENTRYPOINT ["python3", "app.py"]
  

6. COPY :

• Purpose: Copy files/folders from local machine into the image we need to provide source & destination.

• Example:

    COPY ./app /usr/src/app
  

7. ADD :

• Purpose: Like COPY, but also:

  • Can unzip archives automatically.

  • Can download from URL (but not recommended — better to use curl/wget).

• Example:

    ADD app.zip /usr/src/app
  

8. WORKDIR :

• Purpose: Sets the working directory for a container instead of going to container.

• Example:

    WORKDIR /usr/src/app
  

• Notes:

  • No need to use cd in RUN — just set WORKDIR.

9. EXPOSE :

• Purpose: Tells Docker the port the container will listen on.

• Example:

    EXPOSE 8080
  

  Notes:

  • This does not publish the port automatically, it’s just documentation.

10. ENV :

• Purpose: Set environment variables inside the container.

• Example:

    ENV=production
  

Important Docker Commands :-

Now let's look at important Docker commands you must know:

Working with Containers

• Run Container :

    docker run -it --name <container-name> <image-name> /bin/bash
  

  • -d runs in detached (background) mode.

  • -p maps host port to container port.

• List running Containers :

    docker ps
  

• List All Containers (running + stopped) :

    docker ps -a
  

• Stop Container :

    docker stop <container-name>
  

• Remove Container :

    docker rm <container-name>
  

• View Container Logs :

    docker logs <container-name>
  

• Execute Command Inside Running Container :

    docker exec -it <container-name>
  

  • -it makes it interactive.

  • bash opens shell inside container.

Steps to Create a Container Using a Dockerfile :-

1. Create the Dockerfile :

• • Using touch command OR using a text editor (like vim, nano, or Visual Studio Code).

        touch Dockerfile
            # OR
        vim Dockerfile
            # OR
        nano Dockerfile
    

2. Add Instructions to Dockerfile :

• Open the Dockerfile in your editor.

• Add basic instructions.

  • For example:

      FROM ubuntu
      RUN echo "Aditya sharma" >/tmp/testfile
    

3. Create an Image from a Dockerfile :

   What Does "Build an Image" Mean?

   • Building an image means converting your Dockerfile into a ready-to-run Docker image.

   • Docker reads each instruction in the Dockerfile (like FROM, RUN, COPY) and creates an image layer step-by-step.

   • The final result is a custom image you can use to run containers.

Use the docker build Command :

• Command Syntax:

    docker build -t <image-name> .
    docker ps -a                          # To check all containers status
  

  • docker build: Command to build an image.

  • -t: Tag the image with a name and optionally a version/tag.

  • . (dot): Current directory (Docker will search for Dockerfile here).

4. Create and Run a Container from an Image :

   Use the docker run Command

   • Command syntax:

       docker run -it --name <container-name> <image-name> /bin/bash 
       cat /tmp/testfile
     

   • Note : In same docker file if instructions changes then new images can be formed easily.

Codes :