STAC-24657: resolve base image via ARG BASE_IMAGE

[viliakov]

Summary

Simplify base image resolution in the apply-oci-labels composite to rely on a single well-known declaration in the Dockerfile: ARG BASE_IMAGE=<registry/name>:<tag>.

The runtime FROM in every docker-images Dockerfile is now FROM ${BASE_IMAGE} (see StackVista/docker-images#109), so we no longer need to:

• Export every ARG NAME=VALUE declaration from the Dockerfile.
• Grep for the last FROM line, strip --platform= / AS <stage>.
• envsubst the ARG references to reconstruct the base reference.

Instead, the action does a single grep -E '^[[:space:]]*ARG[[:space:]]+BASE_IMAGE=', takes the first match, strips the prefix, and uses the value as org.opencontainers.image.base.name. base-digest resolution via docker buildx imagetools inspect is unchanged.

The base-name input remains an override for build systems (Family B-E) whose Dockerfile does not declare ARG BASE_IMAGE (or has no Dockerfile at all).

Changes

• action.yml - replace FROM-parsing + ARG-export + envsubst block with a single-line ARG BASE_IMAGE= grep. Updated input descriptions.
• testdata/Dockerfile - use ARG BASE_IMAGE=... + FROM ${BASE_IMAGE} to match the new convention.
• apply-oci-labels-ci.yml - comment update only; the matrix and assertions are unchanged. Existing smoke matrix already covers both the Dockerfile-extraction path and the overrides path.

Compatibility

Callers that already pass explicit base-name (Family B-E) are unaffected. Callers that rely on Dockerfile extraction must declare ARG BASE_IMAGE=; the docker-images PR above completes that rollout for the first consumer.