Set cacheCapabilities.digestFunction for ActionCaches

Bazel expects that the execution server will return all the capabilities of all endpoints, rather than just its own. Since BuildGrid endpoints just return their own capabilities, Bazel doesn't work with the default docker-compose layout.

This commit adds cacheCapabilities.digestFunction to the capabilities returned by an ActionCache, since it is relevant to how the cache works. This works around the incompatibility with Bazel without making the Execution endpoint return all capabilities.

This solves issue #174 (closed).