Difference between CMD and ENTRYPOINT in Dockerfile

README.md

Using only ENTRYPOINT

$ docker build --file Dockerfile1 --tag foo/foo1 .
...
$ docker run --rm --tty --interactive foo/foo1
['run.py']
$ docker run --rm --tty --interactive foo/foo1 mine only
['run.py', 'mine', 'only']

Using only CMD

$ docker build --file Dockerfile2 --tag foo/foo2 .
...
$ docker run --rm --tty --interactive foo/foo2
['run.py', 'a', 'bisee']
$ docker run --rm --tty --interactive foo/foo2 python3.7 run.py mine only
['run.py', 'mine', 'only']

Using both ENTRYPOINT and CMD

$ docker build --file Dockerfile3 --tag foo/foo3 .
...
$ docker run --rm --tty --interactive foo/foo3
['run.py', 'a', 'bisee']
$ docker run --rm --tty --interactive foo/foo3 mine only
['run.py', 'mine', 'only']

Using neither ENTRYPOINT nor CMD

$ docker build --file Dockerfile4 --tag foo/foo4 .
...
$ docker run --rm --tty --interactive foo/foo4
Python 3.7.2 (default, Dec 26 2018, 08:50:25)
[GCC 6.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
$
$ docker run --rm --tty --interactive foo/foo4 python3.7 run.py mine only
['run.py', 'mine', 'only']

Dockerfile1

FROM python:3.7-alpine

WORKDIR /app
COPY run.py run.py
ENTRYPOINT ["python3.7", "run.py"]

Dockerfile2

FROM python:3.7-alpine

WORKDIR /app
COPY run.py run.py
CMD ["python3.7", "run.py", "a", "bisee"]

Dockerfile3

FROM python:3.7-alpine

WORKDIR /app
COPY run.py run.py
ENTRYPOINT ["python3.7", "run.py"]
CMD ["a", "bisee"]

Dockerfile4

FROM python:3.7-alpine

WORKDIR /app
COPY run.py run.py

run.py

import sys


def main():
    print(sys.argv)


if __name__ == "__main__":
    main()