"exec format error" for ENTRYPOINT pointing to shell script lacking shebang line

I'm posting this here because error message is raised by a file in this project:

standard_init_linux.go:195: exec user process caused "exec format error"

As an end user I'd like it if somehow it was possible to give a more informative error.

However I have identified what I was doing wrong to cause the error so feel free to close this issue, I mainly wanted it saved somewhere so anyone else having same problem could find the solution in google.

$ docker version
Client:
 Version:      17.04.0-ce
 API version:  1.28
 Go version:   go1.7.5
 Git commit:   4845c56
 Built:        Wed Apr  5 23:33:17 2017
 OS/Arch:      darwin/amd64

Server:
 Version:      17.12.1-ce
 API version:  1.35 (minimum version 1.12)
 Go version:   go1.9.4
 Git commit:   7390fc6
 Built:        Tue Feb 27 22:20:43 2018
 OS/Arch:      linux/amd64
 Experimental: false

What happened is I had a Dockerfile like:

FROM python:3.6-slim

...

ENTRYPOINT ["pipenv", "run", "celery", "worker"]

Then I wanted to add a "--loglevel=$LOGLEVEL" segment to the entrypoint, but env vars are not interpolated there. So I found the advice to move my startup commands into a shell script and use that as my ENTRYPOINT instead.

I created an entrypoint.sh containing:

pipenv run celery worker --loglevel=$LOGLEVEL $@

and set file permission to executable.

But then when I docker run I get

standard_init_linux.go:195: exec user process caused "exec format error"

This is puzzling because if I manually run ./entrypoint.sh from the command line it works

By trial end error I found that adding the shebang, i.e.

#!/bin/bash
pipenv run celery worker --loglevel=$LOGLEVEL $@

fixed the problem.

I guess this is something to do with how the exec process works vs running in a terminal shell session.

Googling for this error message turned up some quite disparate causes, eg

• several issues in moby project like https://github.com/moby/moby/issues/35447 caused by mismatch in architecture of binaries vs docker host
• this blog post https://blog.hompus.nl/2018/01/24/docker-run-throws-exec-user-process-errors/ where seemed like changing encoding of file away from utf8 fixed problem

I wonder if the error message can be more informative or specific to separate problems.

Start by inspecting standard_init_linux.go around line 195 and tracing how the exec format error is propagated. Compare the reported shebang-less script case with the other causes mentioned, including architecture mismatch and file encoding. The issue does not define a specific diagnostic or acceptance criteria, so the desired behavior would need agreement before implementation.