Add a custom Android CI/release build image and packaging scripts

- build-image/: Dockerfile bundling JDK 17, the Android SDK, and
  Node.js (needed by Gitea's JS-based actions) - the app has no
  native code, so no NDK
- build-image.sh/upload-image.sh: extended to also build/push this
  image alongside the existing OCR pipeline tools image
- run-image.sh: runs `make android` inside the image against a live
  checkout, for a CI-equivalent local build
- Makefile: `make android` builds and versions a release APK into
  dist/, from the current git tag
- .gitea/workflows/build.yml: now pulls the custom image and just
  runs `make android`, replacing the ad-hoc Node/SDK install steps
- README.md: describes the pipeline, server, app, and build tooling
- registry.env.example: fix REGISTRY_IMAGE to be fully-qualified
  (an unqualified name was silently resolving pushes to Docker Hub
  instead of the configured registry)

.gitignore

@@ -18,6 +18,9 @@
 __pycache__/
 *.pyc
 
+# Build output (see the root Makefile's `android` target, run-image.sh)
+/dist/
+
 # Android app build output/local config and generated resources (launcher
 # icon + splash art - see android/app/build.gradle.kts's
 # generateAppIcons task and android/scripts/gen_app_icons.py's own
@@ -32,3 +35,7 @@ __pycache__/
 /android/app/src/main/res/mipmap-xhdpi
 /android/app/src/main/res/mipmap-xxhdpi
 /android/app/src/main/res/mipmap-xxxhdpi
+
+# Android Studio project files - per-developer, not shared
+/android/.idea/
+/android/gradle/gradle-daemon-jvm.properties