Surface deploy parameters (--set / --set-value-file) to exec custom-action and verify containers for Cloud Deploy parity #10128

Problem

Google Cloud Deploy passes deploy parameters to its execution environment as environment variables, surfacing them to render, custom action, and verify containers. Skaffold already accepts deploy parameters via --set / --set-value-file, but historically they were only used for manifest templating and only exposed on a subset of commands (render, filter, delete). As a result:

• skaffold exec (custom actions) did not inject deploy parameters as environment variables into action containers.
• skaffold verify did not accept --set / --set-value-file at all, and never injected deploy parameters into verify containers.

This breaks Cloud Deploy parity. A custom action (e.g. an Alembic database migration) cannot read a Cloud SQL connection URL passed as a deploy parameter, and a post-deploy verify smoke test cannot read the deployed Cloud Run service URL that Cloud Deploy injects automatically.

Expected behavior

--set / --set-value-file should be available on deploy, dev, run, exec, and verify (in addition to the existing render / filter / delete). On exec and verify, the supplied key/value pairs should additionally be injected as environment variables into every custom-action / verify container, with precedence (lowest → highest):

1. --env-file (base)
2. --set-value-file
3. --set

Current behavior

• skaffold verify rejects --set / --set-value-file (flags are not registered on the command).
• exec / verify containers receive no deploy parameters as environment variables.

Reproduction

Verify (Cloud Run service URL → Playwright smoke test):

# skaffold.yaml
apiVersion: skaffold/v4beta14
kind: Config
metadata:
  name: repro
verify:
  - name: smoke-test
    container:
      name: smoke
      image: alpine:3.15.4
      command: ["/bin/sh", "-c"]
      args: ["echo \"smoke test against ${SERVICE_URL}\""]

$ skaffold verify --set SERVICE_URL=https://my-service-abc-uc.a.run.app
Error: unknown flag: --set        # today
# expected: the container prints "smoke test against https://my-service-abc-uc.a.run.app"

Exec (Cloud SQL URL → Alembic migration):

# skaffold.yaml
apiVersion: skaffold/v4beta14
kind: Config
metadata:
  name: repro
customActions:
  - name: db-migrate
    containers:
      - name: alembic
        image: alpine:3.15.4
        command: ["/bin/sh", "-c"]
        args: ["echo \"alembic upgrade head -> ${DATABASE_URL}\""]

$ skaffold exec db-migrate --set DATABASE_URL=postgresql+pg8000://user:pass@/app?unix_sock=/cloudsql/PROJECT:REGION:INSTANCE/.s.PGSQL.5432
# today: DATABASE_URL is empty inside the container
# expected: the container prints the full connection URL

Proposed solution / scope

• Expose --set / --set-value-file on deploy, dev, run, exec, and verify.
• Inject the merged deploy parameters as environment variables into custom-action containers (exec) and verify containers (verify), reusing one shared merge helper with the precedence above.
• Add runnable examples (custom-action Cloud SQL/Alembic migration; Playwright verify smoke test) and document the behavior on the custom-actions and verify pages.