Speed up connector builds by caching container layers

[gileri]

Use case

There is now a lot of containers, and those take 2 * 24 minutes to build. The same package/files are pulled from upstream Alpine repos. Each image push push the whole container, instead of a few or even no changes to Docker hub. All those are run at each change on the main branch.

Current Workaround

Currently two builds are run through CircleCI with half the connectors each (build_rolling_1 and build_rolling_2) to speed it up a bit.

Proposed Solution

Refactor Dockerfiles to allow caching of most layers, and enable it on CircleCI.

Additional Information

Would you be willing to submit a PR?

I've started one here. Should we continue like that ?