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)

Makefile

@@ -0,0 +1,45 @@
+# Builds the Android app (android/) and packages a versioned release APK.
+# The OCR pipeline itself (tooling-container/, 0-3_*.sh, test_batch.sh) is
+# container-only and has no host build step, so this Makefile only covers
+# the Android app.
+
+ANDROID_DIR := android
+ANDROID_APK := $(ANDROID_DIR)/app/build/outputs/apk/debug/app-debug.apk
+DIST_DIR    := dist
+
+.PHONY: all android clean
+
+all: android
+
+# Builds the Android app (`./gradlew assembleDebug` in android/, via its
+# own Gradle/AGP toolchain) and copies the resulting debug APK into
+# dist/, named with the current git tag, or `make android VERSION=1.2.3`
+# to override. Untagged builds (including every CI build, unless a
+# release tag is pushed) get a 0.0.0-dev+<sha> placeholder version.
+# Ships the auto-generated debug keystore, not a release signing config -
+# fine for sideloading; a real release signing setup is a separate,
+# not-yet-needed step. Requires the Android SDK (ANDROID_HOME set) - see
+# build-image/Dockerfile for the exact version this is built/tested
+# against, or run this inside it via ./run-image.sh.
+android:
+	cd $(ANDROID_DIR) && ./gradlew assembleDebug
+	@mkdir -p $(DIST_DIR)
+	@V="$(VERSION)"; \
+	if [ -z "$$V" ]; then \
+		if TAG=$$(git describe --tags --exact-match --match 'v[0-9]*.[0-9]*.[0-9]*' 2>/dev/null); then \
+			V=$${TAG#v}; \
+		else \
+			V="0.0.0-dev+$$(git rev-parse --short HEAD)"; \
+			echo "WARNING: HEAD is not on a vX.Y.Z tag - building placeholder version $$V (push a tag to drive a real release version)" >&2; \
+		fi; \
+	fi; \
+	case "$$V" in \
+		[0-9]*.[0-9]*.[0-9]*) ;; \
+		*) echo "PREFLIGHT FAIL: VERSION '$$V' is not a semantic version (expected X.Y.Z, optionally with a -pre+meta suffix)" >&2; exit 1;; \
+	esac; \
+	APK_NAME="fuelish-$$V.apk"; \
+	cp $(ANDROID_APK) "$(DIST_DIR)/$$APK_NAME"; \
+	echo "Wrote $(DIST_DIR)/$$APK_NAME"
+
+clean:
+	rm -rf $(DIST_DIR) $(ANDROID_DIR)/build $(ANDROID_DIR)/app/build