POD-823: Add repo cache

[bkneis]

This PR introduces a new CLI parameter --registry-cache to toggle the kaniko builder's cache from a local directory to a registry.

Testing on my local kind cluster I have run through two workflows, after building devpod's examples/build (see loft-sh/devpod#1245):

1. Adding a devcontainer.json feature (docker in docker)

• With cache: 3m21s
• Without cache: 7m36s

2. Changing build context (adding files to examples/build/app and adding RUN layer to Dockerfile)

• With cache: 3m13s
• Without cache: 7m46s

As you can see the remote cache has managed to save a user 50% build time when building updates to an existing image. Unfortunately the overall build time is still not great and this is a known issue - GoogleContainerTools/kaniko#875

We could switch to docker in docker, but this comes with security concerns since we will need priv permissions to access the daemon.sock :/

EDIT: Some users report http2 being the cause, GoogleContainerTools/kaniko#2751. I will try implement this and see if it improves build time.

[pascalbreuninger]

LGTM! Do you want to open another PR for the http2 fix or do it here?

[bkneis]

@pascalbreuninger I'll do it here and continue to use this PR until tomorrow once testing has been complete :) So far though I have managed to get build times closer to 2 minutes by optimizing the snap shotting and skipping pushing of cache :) Once I can get the prebuild hash to play with dockerless then kaniko will be close to dind speed!