We stick as close as feasible to "POSIX shell", but we do expect a very few extra things. Not big features, just sane features:

1. We expect local to exist, as a builtin, to declare variables local inside a function.
2. We expect that source (or .) pass parameters to the sourced file in argv.

Almost all POSIX or POSIX-alike shells comply with these two feature requirements. But dash does not pass arguments to source. And dash is /bin/sh on a variety of Linux-derived systems.

The /bin/sh in Alpine is busybox, using ash. Despite dash being derived from ash, in this case ash supports passing arguments. And Alpine does not include bash by default, so we can't just write #!/bin/bash.

So:

1. If the script is to be run inside the container, then #! should reference /bin/sh
2. If the script is to be run from a host machine, or otherwise outside the container, then #! should reference /bin/bash.