Dockerfile Entrypoint: “file not found”

I was working with a fairly simple Dockerfile, defining an entrypoint and always got a “not found” error when trying to run the container.

My Dockerfile looked something like this:

FROM python:3.9-alpine

[...]

WORKDIR /app
COPY . /app

RUN ["chmod", "+x", "./entrypoint.sh"]
ENTRYPOINT ['./entrypoint.sh']

Building the image worked without issues, but running the container was giving me the mentioned error:

$ docker build -t what/ever:latest
[...]
$ docker run --rm what/ever:latest
/bin/sh: [./entrypoint.sh]: not found

I made sure that my entrypoint.sh file actually existed by overriding the entrypoint and inspecting the container:

$ docker run -it --rm --entrypoint sh what/ever:latest
/app # ls -l
total 28
-rwxr-xr-x    1 root     root           137 Nov  7 19:32 entrypoint.sh
[...]

The file existed at the expected location, so what’s the issue?

It’s the quotes!

After verifying my Dockerfile again, I saw my – in retrospect – obvious mistake: the quotes!

Since the ENTRYPOINT list is parsed as a JSON array, it needs to be in double-quotes, not single-quotes.

Adjusting the Dockerfile like follows fixes the issue:

# before
ENTRYPOINT ['./entrypoint.sh']

# after
ENTRYPOINT ["./entrypoint.sh"]

In case the entrypoint.sh script would really be missing, the error would also be different than the one above. You would rather get something more descriptive like:

starting container process caused: exec: "./entrypoint.sh": stat ./entrypoint.sh: no such file or directory