Cache Docker layers and cargo workspace in Actions builds

[yourbuddyconner]

Currently from-scratch builds of the agents take approximately 25 minutes, this can be sped up by adding a cache step to our docker build workflow.

actions/cache Rust example

[ErinHales]

So glad you added this issue already. Had noticed this was taking a long time :)

[yourbuddyconner]

This is going to be slightly more complicated than I anticipated, details here: https://gist.github.com/UrsaDK/f90c9632997a70cfe2a6df2797731ac8

[yourbuddyconner]

Alternatives that would let us keep the docker/build-push action: https://github.com/docker/build-push-action/blob/master/docs/advanced/cache.md