dockerfile

dockerfile is the recipe to build a docker image

• FROM → specify name of base image
• WORKDIR <path> → change workdir, all following instructions will be executed in the given location.
• COPY <source> <dest> → copy files from host to container.
• RUN command → run commands
• CMD [...] → command to run when the container is started.

Here is an example dockerfile to run a shell script.

FROM alpine:latest
WORKDIR /usr/src/app
COPY ./hello.sh .
RUN chmod +x ./hello.sh
CMD ./hello.sh

To build the image, we use the following command.

docker build LOCATION-OF-DOCKERFILE -t CONTAINER-NAME

CMD vs ENTRYPOINT

Both CMD and ENTRYPOINT take a command, that is run when the container starts. But if we pass a command while running a container, it will replace the CMD command.

CMD ["echo"]
---
docker run -it NAME ps -a  # this will replace the CMD command

But if you use ENTRYPOINT, the command passed while running a container will instead become the arguments of the entrypoint.

ENTRYPOINT ["echo"]
---
# "hello" becomes arguments of the entrypoint command
# effectively running echo "hello"
docker run -it NAME "hello" 

If you pass both CMD and ENTRYPOINT, CMD will become the default arguments of the ENTRYPOINT.

CMD ["hello"]
ENTRYPOINT ["echo"]
---
docker run -it NAME # output: "hello"
docker run -it NAME "hi" # output: "hi"

related

contents

• basic docker commands
• running docker containers
• dockerfile
• bind mount, ports and expose
• docker volumes
• docker-compose
• docker networking

creating container from scratch → https://youtu.be/8fi7uSYlOdc namespaces, cgroups, union filesystems

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