buildctl: define oci-layout flag

[jedevc]

This ensures that the new oci-layout can be loaded using buildctl (see #2827 (comment)).

Example usage:

buildctl --addr tcp://localhost:1234 build \
  --frontend=dockerfile.v0 \
  --local context=. --local dockerfile=. \
  --opt context:foo=oci-layout://foodir@sha256:79a06bdbfbf27b365be61ca38646b07b451681475c284ad79deed506ab192917 \
  --oci-layout foodir=./path/to/oci/dir

Previously, the --oci-layout flag was not registered with cobra, so could not be provided correctly.

CC @deitch @tonistiigi

[deitch]

@jedevc so how does this get used from the CLI? Or more specifically, what are the steps I need? It looks like:

1. Place the context in the Dockerfile, eg. FROM foo or FROM alpine:3.16 as foo
2. Map the path to a named dir, in your example --oci-layout foodir=./path/to/oci/dir
3. Map the context from the Dockerfile to the named dir, in your example --opt context:foo=oci-layout://foodir@sha256:79a06bdbfbf27b365be61ca38646b07b451681475c284ad79deed506ab192917

The above 3 steps will map foo to the oci layout named foodir, pulling the specific index/manifest with that hash, and finding foodir by looking up what was in --oci-layout foodir=...

Is that it?