Per-repository cache for Dockerfile as a CI environment

[fkorotkov]

Description

Add a repository setting to enable per-repository caching for docker images instead of the default global one. Make new repositories to use the per-repository caching as it's safer from security perspective.

Context

Right now Dockerfile as a CI environment is cached globally. Because of #273 it is possible to inject malicious scripts, then remove it so a PR will look harmless but the cached image will still contain the malicious code. It's documented in #760.

#273 and #692 can help to lower the risk but they won't completely mitigate the potential vector of attack.

[fkorotkov]

Now images are cached per repository in the format of

cirrus-ci-community/${repository.fullNane}/${dockerfile.path}:${dockerfile.fingerprint}

While building the image we've added --cache-from flag to use the legacy globally cached image if any. This should minimize the disturbance of the change by making the rebuilt just copy over the legacy image to the new location.