only build docker boxes on master

[thedavidmeister]

PR summary

testing/benchmarking notes

( if any manual testing or benchmarking was/should be done, add notes and/or screenshots here )

followups

( any new tickets/concerns that were discovered or created during this work but aren't in scope for review here )

changelog

• [ ] if this is a code change that effects some consumer (e.g. zome developers) of holochain core, then it has been added to our between-release changelog with the format

- summary of change [PR#1234](https://github.com/holochain/holochain-rust/pull/1234)

documentation

• [ ] this code has been documented according to our docs checklist

[zippy]

I want to hold off on merging this, because these docker box builds are what we use in final-exam tests and so I don't want to wait for a release to build them. I pretty much restart the pool every time we have a merge to develop. I could get some practice where I merge develop into final-exam, but that's pretty much equivalent and doesn't save us any build time.

[thedavidmeister]

@zippy it also builds on the final-exam branch as well as master is that not right any more?

[thedavidmeister]

@zippy or, rather, i'm not sure that having all the boxes build on develop achieves what you're wanting anyway... we should have a quick sync on this f2f