feat(container): add UBI 10 image with scan-gated GHCR publish workflow

[jonpspri]

Summary

Delivers the F3 container-image work for mcp-reverse-proxy:

• Container image — Containerfile (UBI 10 builder/runtime multi-stage), pinned requirements-container.txt (353 locked deps), and a minimal .dockerignore build context.
• Secure publish workflow — .github/workflows/container.yml: scan-gated, immutable-tag publish to ghcr.io/contextforge-org/mcp-reverse-proxy with Trivy scanning, Buildx provenance, and Sigstore signing; publish is restricted to main pushes and v* tags only.
• Buildx Sigstore verification — the Buildx bundle (provenance attestation) was verified locally with gh attestation verify-equivalent Sigstore checks; CI reproduces the same attested build.
• Docs — README.md and USER_README.md document published-image usage.

Three commits, all DCO-signed:

• c9e9e92 feat(container): add UBI 10 builder-runtime Containerfile
• 3296d50 ci(container): add scan-gated ghcr.io publish workflow
• 0009441 docs(container): document published image usage

Publish guard

The publish job is guarded off for pull requests:

if: github.event_name != 'pull_request' && (github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/v'))

PRs cannot run publish. On pull_request only Build + smoke test (no push) executes — no registry login, no push, no signing, no tagging.

Testing (local QA, all passed)

• Buildx bundle verification — Buildx Sigstore/provenance checks passed locally against the built bundle.
• actionlint — workflow file lints clean.
• Trivy — image scan completed (pinned aquasec/trivy@sha256:62b1e6…, v0.74.0).
• Image smoke — container build succeeded; all seven smoke assertions passed.
• External-copy pytest — test suite run against an external copy of the repo passed.
• DCO — all three commits carry Signed-off-by trailers.

Independent reviewers F1, F2, and F4 returned APPROVE on this branch.