Building from source: six small PRs and some questions

[naanlizard]

We run a fork of OME in production and build it from source. We've found some bugs (we think) along the way. We opened a few small PRs:

• #2294 parallelize the
  openh264 build. Every other dependency builds with the parallel job count; openh264 is
  serial.
• #2295 add a .dockerignore.
  The build context ships .git, which changes on every commit, so no layer after
  COPY . ... ever caches.
• #2296 link the split ggml
  archives with static whisper.
• #2297 require Clang 21 or
  newer for OME_THREAD_SAFETY. The annotations need 21; the installer provides the
  distribution's Clang, which is 14 on the Dockerfile's own base image.
• #2298 stop the compiler
  override from wiping the CMake cache. A second configure can delete the cache and re-run
  with defaults, so a build asking for Release comes out Debug.
• #2301 stop installing clang
  when it was not asked for. Reinstalling a single dependency also apt-installs a compiler.

And some stuff that I don't think I know what to do about, so I'm just leaving a message here.

• The published images build with GCC instead of clang - this seems like a bug
• There ARE tests, but github doesn't run them in CI, I assume that the private version of the repo does that locally or something?
• With Clang 21 or 22 and -DOME_THREAD_SAFETY=ON, master builds and the analysis reports 14 warnings, all in src/transcoder/. We have not tried to judge them. Given the weirdness with building, it seems plausible that they might be silently ignored somehow.