Add CI workflow that builds the podman image

[nikicat]

Adds a GitHub Actions workflow that builds the container image with podman and fixes the image build, which was already broken on master.

What's here

.github/workflows/build-image.yml (new)

• Triggers on push to master, pull requests, and manual dispatch.
• PR / manual: make image with --layers --cache-from — build-only, no push.
• master: make push with --layers --cache-from --cache-to — builds, populates the registry cache, and pushes the image + tools image to ghcr.io/nikicat/tg-echo-service.
• Auth via the built-in GITHUB_TOKEN (no secrets to add). Includes recursive submodule checkout, a disk-cleanup step (the WebRTC build is large), concurrency cancel of superseded runs, and a 6h timeout.

Containerfile — adds COPY video_platform.cpp .. Commit 935f6a7 added that file to the tgcalls target in CMakeLists.txt but never to the Containerfile, so make image failed at configure time. Placed after the TDLib build so editing it doesn't bust the cached TDLib layer.

Makefile — adds an IMAGE_BUILD_ARGS ?= hook injected into the image target's podman build. Empty for local make image; CI passes the cache flags through it, so the build command stays single-sourced.

Why registry cache (not type=gha)

podman has no GHA cache backend; cross-run caching is registry-backed only (podman ≥4.2 --cache-to/--cache-from). Since we already push to GHCR, the cache lives at ghcr.io/nikicat/tg-echo-service/buildcache and keeps the expensive TDLib/tgcalls/WebRTC layers warm between runs.

Notes

• First master run is a cold build (no cache yet) and creates the GHCR packages (private by default — flip to public in repo settings to allow anonymous pulls).
• Fork PRs get a read-only token and can't read the private cache, so they fall back to a full build (no hard failure). Same-repo PRs use the cache.
• The heavy build wasn't run locally; the Containerfile fix is from static analysis of the CMake source list, and this workflow run is the end-to-end check.

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