Improve build time

[felipecrs]

By implementing cache, as suggested by @nanosonde at #112 (comment).

[felipecrs]

Yes, we can implement cache. But the current Dockerfile isn't very cache friendly, we should ideally pin the version of all the apt-get dependencies.

#112 (comment)

[felipecrs]

It seems that the action we use already has builtin cache:

https://github.com/home-assistant/builder#publish-action-example

But it's only valid for merges to master, not for PRs or other branches.

In order to implement cache for all cases, we will need to switch away from the addon-builder to docker/build-and-push instead.

[felipecrs]