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Fixes #958

Problem outline

As it currently stands, you can not test source files which have #!/bin/bash at the top of them inside the bats docker image. This is because bash is not located here.

I am aware that it is better practice to have #!/usr/bin/env bash as the shebang at the top of bash scripts - to make them more portable.
However, the vast majority of bash code on GitHub uses /bin/bash (~8 million files). Which means, at them moment, they can't be tested inside the docker image provided by bats without changing their source code.

Summary of change

My proposed change is to symlink bash to the /bin/bash location - which means bats can be used (specifically the docker container running of the tests) can be used by a much wider group of source files.

Testing done

I've done some minimal testing. I've built the docker image using docker build --build-arg bashver="5" --tag bats/bats:bash-pdruce . and then I tested that I could access /bin/bash with the command docker run -it --rm --entrypoint /bin/bash bats/bats:bash-pdruce.

This currently doesn't work for bats/bats:latest - docker run -it --rm --entrypoint /bin/bash bats/bats:latest