e2e: test full function of custom shebangs in definition files

[dtrudg]

Type of issue technical debt

Description of issue

The e2e suite does not test the function of custom shebangs in runscripts (or test scripts with #868).

We have parsing unit-tests that ensure the parsed script is correct, but it would be nice to have some e2e coverage exercising it fully.

[dtrudg]

Through the work on #872 it's apparent that there's an easy way to do this on an alpine image calling /bin/busybox sh in its script hashbang, and then inspecting /proc/$$/cmdline

Should be a simple addition now this is identified.