Allow using a remote registry for caching

[sambhav]

As dicussed in slack at https://buildpacks.slack.com/archives/CQ10WHMT4/p1613732146033900

TL;DR -

Platforms like pack allow storing the cache layers in a remote registry. kpack should allow doing the same for clusters that don't have PVCs or for more flexibility with caching in general. This might require a spec change from spec.cacheSize to spec.cache which looks like -

Something like:

spec: 
  cache: 
    volume: 
      request: 2G
      storageClassName: some-class-name

and:

spec: 
  cache: 
    registry: 
      tag: myregistry.com/image/tag

[divoxx]

It would also be great if the two caching strategies can be used in conjunction. i.e. being able to have local builds as fast as possible, while still allowing some level of caching for multi-clusters setup, or when the cache needs to be busted.

[matthewmcnew]

It would also be great if the two caching strategies can be used in conjunction. i.e. being able to have local builds as fast as possible, while still allowing some level of caching for multi-clusters setup, or when the cache needs to be busted.

@divoxx Interesting, idea! I think that might require a change to the underlying cloud native buildpack project to either support multiple types of cache or some cache hierarchy. Would you be interested in bringing that up in Buildpacks slack or Buildpacks working group?