Consider switching out bash requirement for sh

[kofalt]

We use bash for a few things internally, and also as the shell within which both the old and new command feature is executed. This is usually fine - scientific algorithms are generally orders of magnitude larger than a basic ubuntu image - but that does mean that we can't use an image like python:alpine, instead using python:slim.

I cannot offhand think of anything we need in bash that isn't in sh, but making the change is tricky. We could offer an opt-in for sh, but gear development tools would have to detect that bash is missing. Or, we could offer an out-out of sh, but that is backwards incompatible, which I am strongly against. We might be able to get away with that at the present time, but not too far into the future.

[kofalt]

Another option: an optional shell key that can be set to bash or sh, defaulting to bash if not present. That's probably a superior approach, but still leaves the detection problem.

I'm not yet convinced we should do this, and am leaning towards no, but leaving this open if this becomes important enough to chase later.

[gsfr]

I don't have the full picture, but I'm strongly in favor of enabling Alpine-based gears. If we're not getting value out of bash, let's get rid of it asap. We probably have only a few more weeks before there are many gears that will be beyond our control. Please ping me to discuss.