by: Programming

Having a custom image would allow one to scale applications on-demand via orchestration (Kubernetes, Rancher, Kasm, etc) because they can be span up fin milliseconds straight from an image.

MAIN STEPS

  • Create a new directory,
  • Create a new file in that directory called Dockerfile,
  • Add the following blocks to the new file:
    • FROM to use an specific basic image.
    • RUN to execute commands during the image build process.
    • COPY / ADD to copy files or directories to the image.
    • CMD to specify the command that will be executed when a container is run from the image.
  • Build the new image (with docker build and -t to give it a name).

There are additional instructions that can be added as needed, such as:

  • ENV to set environment variables in the image.
  • EXPOSE to declare what ports the container will listen at runtime.
  • USER is used to set the UID and/or GID that the command, run by CMD, should be executed as.
  • WORKDIR defines the working directory for subsequent instructions, such as RUN and CMD.
  • VOLUME creates a persistent volume in the image (to survive container restarts).
  • and more.

Note: COPY and ADD are not the same. Essentially, COPY only copies local files and directories from the build context to the image. While ADD also supports automatically extract files from a compressed archive (such as a tar or zip file) and to copy files from a remote URL.

EXAMPLES

For the Dockerfile:

FROM alpine:latest
COPY ./hello.sh /usr/local/bin/hello
RUN chmod +x /usr/local/bin/hello
CMD ["/usr/local/bin/hello"]
FROM python:3.9-alpine
ENV PYTHONUNBUFFERED 1
COPY . /app
WORKDIR /app
RUN pip install --upgrade pip
RUN pip install -r requirements.txt
EXPOSE 80:5000/tcp
CMD ["flask", "run", "--host=0.0.0.0"]
FROM postgres:12
ENV POSTGRES_DB dbHost
ENV POSTGRES_USER dbUser
ENV POSTGRES_PASSWORD dbPassword
COPY init.sql /docker-entrypoint-initdb.d/
EXPOSE 5432

For the build:

sudo docker build -t customImageName .

EXTRACT IMAGE CONTENT

Sometimes it is needed to extract the content of a Docker Image for analysis.

sudo docker save USER/IMAGE > USER_IMAGE.tar
tar xvf USER_IMAGE.tar

Or from a Docker Container:

sudo docker export CONTAINER > CONTAINER.tar
tar xvf USER_IMAGE.tar