Build the Android app on our own Gitea runner

Pushing a v* tag builds the normal and FOSS APKs, signed with the upload
key, and attaches them to a Gitea release; a manual run on a branch
uploads a test build to the repo's packages. Everything runs on the
Gitea server: a builder image with the Android SDK, an act_runner
registered only for this repo, and plain shell steps (no third-party
actions). The build itself is apps/mobile/scripts/build-android.sh, the
same script for hand builds. Setup steps are in docs/ANDROID-CI.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/android-builder/Dockerfile

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+# Build environment for the Android app, used by the Gitea Actions workflow
+# (.gitea/workflows/android.yml). Build it on the machine running the runner:
+#
+#   docker build -t musicshare-android-builder deploy/android-builder
+#
+# Versions match what React Native 0.86 / Expo 57 ask for
+# (node_modules/react-native/gradle/libs.versions.toml). Gradle downloads anything
+# else it needs on first use; the workflow keeps ~/.gradle in a volume.
+FROM node:22-bookworm
+
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends openjdk-17-jdk-headless unzip \
+ && rm -rf /var/lib/apt/lists/*
+
+ENV JAVA_HOME=/usr/lib/jvm/java-17-openjdk-amd64 \
+    ANDROID_HOME=/opt/android-sdk \
+    ANDROID_SDK_ROOT=/opt/android-sdk
+ENV PATH=$PATH:$ANDROID_HOME/cmdline-tools/latest/bin:$ANDROID_HOME/platform-tools
+
+# Checksum from https://developer.android.com/studio#command-line-tools-only
+ARG CMDLINE_TOOLS=commandlinetools-linux-15859902_latest.zip
+ARG CMDLINE_TOOLS_SHA256=4e4c464f145a7512b57d088ac6c278c03c9eea610886b35a5e0804e74eedf583
+RUN curl -fsSLo /tmp/tools.zip https://dl.google.com/android/repository/$CMDLINE_TOOLS \
+ && echo "$CMDLINE_TOOLS_SHA256  /tmp/tools.zip" | sha256sum -c \
+ && mkdir -p $ANDROID_HOME/cmdline-tools \
+ && unzip -q /tmp/tools.zip -d $ANDROID_HOME/cmdline-tools \
+ && mv $ANDROID_HOME/cmdline-tools/cmdline-tools $ANDROID_HOME/cmdline-tools/latest \
+ && rm /tmp/tools.zip
+
+RUN yes | sdkmanager --licenses > /dev/null \
+ && sdkmanager --install \
+      "platform-tools" \
+      "platforms;android-36" \
+      "build-tools;36.0.0" \
+      "ndk;27.1.12297006" \
+      "cmake;3.22.1" > /dev/null \
+ && chmod -R a+rwX $ANDROID_HOME