fix(docker): pin base to ubuntu:24.04 to fix broken build

ubuntu:latest rolled to 26.04 LTS on 2026-05-06, breaking the Docker
build for 50+ consecutive CI runs. Playwright 1.58.0 (pinned in uv.lock)
cannot install firefox deps for ubuntu26.04-x64 -- it prints 'Cannot
install dependencies for ubuntu26.04-x64 with Playwright 1.58.0!' and
installs nothing, leaving libgtk-3.so.0 absent. Camoufox's bundled
Firefox then fails to load XPCOM at runtime:

  libgtk-3.so.0: cannot open shared object file: No such file or directory
  Couldn't load XPCOM.

Pinning to 24.04 (the last-known-good base, supported by Playwright 1.58)
restores libgtk-3-0t64 and the rest of the GTK runtime. Adopted from PR #362
which independently diagnosed the same issue.

Verified locally:
- app stage: ldconfig shows libgtk-3.so.0 present (was absent)
- test target: 6/6 tests pass (was BrowserType.launch failure)
- runtime: POST /v1 returns 200 status:ok (was 500 libgtk-3 traceback)

Dockerfile

@@ -1,5 +1,7 @@
-# Ubuntu is required by playwright
-FROM ubuntu:latest AS base
+# Ubuntu is required by playwright.
+# Pin to 24.04 LTS: ubuntu:latest floats to 26.04, which Playwright 1.58
+# cannot install firefox deps for (no libgtk-3 -> camoufox fails to launch).
+FROM ubuntu:24.04 AS base
 
 ARG GITHUB_BUILD=false \
     VERSION