build(app): add Android setup with Docker

app/CONTRIBUTING.md

@@ -58,3 +58,17 @@ flutter pub run flutter_launcher_icons:main
 ```
 
 This will generate icons for both iOS as well as Android.
+
+## Building with Docker for Android
+
+Using `instrumentisto/flutter` to build (of course also possible locally, but
+might not work due to conflicting Java/Gradle/... versions).
+
+Add keys according to
+[Flutter docs](https://docs.flutter.dev/deployment/android) and set the path in
+`key.properties` to `/keystore/upload-keystore.jks` (this is where the
+to-be-mounted volume points to; would recommend to keep the original path as a
+comment for reference).
+
+Execute the `build_android.sh` script:
+`bash build_helpers/build_android.sh <path-to-keystore> <path-to-repository>`.

app/build_helpers/android.Dockerfile

@@ -0,0 +1,7 @@
+FROM instrumentisto/flutter:3.7.6
+COPY . /app
+RUN rm -rf /app/build
+WORKDIR /app
+RUN dart pub get
+RUN flutter pub get
+RUN flutter pub run build_runner build --delete-conflicting-outputs

app/build_helpers/build_android.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+
+# Run this from the app project root
+# Receives path to keystore and path to repository as parameters
+
+output_path="app/build/app/outputs/bundle/release"
+docker build -f build_helpers/android.Dockerfile -t build-pharme-for-android .
+docker run \
+    -v $1:/keystore \
+    -v $2/$output_path:/$output_path \
+    build-pharme-for-android \
+    flutter build appbundle