Speed up CI docker build with caching

[zone117x]

Initial attempt in PR https://github.com/hirosystems/stacks-blockchain-api/pull/901

Currently the CI docker build step takes 15-20 minutes. Most PRs only modify the typescript source, which means most of the time only the npm run build step should be required to run without caching, which takes less than a minute.

The other docker file commands could leverage the caching feature provided by the docker/build-push-action action. For example here are some lengthy steps that could easily be cached:

• apk add ...,
• npm install ...
• Building the stacks-node from source