Cache Bitcoin Core docker image

[edilmedeiros]

After adding a CI job in #13, I noted that most of the time is spent compiling Bitcoin Core itself.
This can be cached to make it faster since we are not messing with Core itself, but with tools around it.

[m4ycon]

Seems to be possible but with some limits, that still fits well for this project. After 7 days they delete the cache and we have a limit of 10GB per repo.

https://docs.docker.com/build/cache/backends/gha/
https://docs.github.com/en/actions/reference/dependency-caching-reference#usage-limits-and-eviction-policy