Dockerfile RUN command return an exec format error

I am trying to create and image from my Python Flask app, but I am still getting this format error and I have no idea why. (I am running it on a Raspberry with Raspbian)

Step 4/5 : RUN echo "test" —> Running in ca7b48ab3d4e
standard_init_linux.go:211: exec user process caused "exec format

The command ‘/bin/sh -c echo "test"’ returned a non-zero code: 1

Here is my Dockerfile

#!/usr/bin/env bash

FROM python:3.6.1-alpine
WORKDIR /project
ADD . /project
RUN echo "test"
CMD ["python3",""]

I think that there is a problem with bash, because it says, that it calls this command with sh, but I do not know, what I am missing.

This echo command will be replaced by RUN pip3 install -r requirements.txt to install dependencies.

Thanks for your help.

Source: Docker Questions