Prebuild in docker image based on local copy of repo

Instead of checking out the default branch. Otherwise it's impossible to
test local changes.

docker/Dockerfile

@@ -47,7 +47,7 @@ ENV GOCACHE /opt/gocache
 ENV GOPATH /opt/gopath
 
 # Run prebuild script (will prebuild stuff into the image if env var is set)
-ADD prebuild.sh /opt/prebuild.sh
-RUN /opt/prebuild.sh
+ADD . /opt/syncthing-android
+RUN /opt/syncthing-android/docker/prebuild.sh
 
 WORKDIR /mnt

docker/README.md

@@ -1,7 +1,15 @@
 # How to use this
 
-1. Build the docker image: `docker build -t syncthing-android-builder:latest .`
-2. Checkout syncthing-android somewhere (for the sake of discussion let's say /tmp/syncthing-android)
-3. Inside /tmp/syncthing-android, do `git submodule init; git submodule update`
-4. Run `docker run --rm -v /tmp/syncthing-android:/mnt syncthing-android-builder ./gradlew buildNative assembleDebug`
-5. Retrieve APKs from /tmp/syncthing-android/app/build/outputs
+## Create the builder image
+
+From inside the checked out syncthing-android repository, run:
+
+`docker build -t syncthing-android-builder:latest -f ./docker/Dockerfile .`
+
+## Build the app
+
+1. From inside the checked out syncthing-android repository, run:
+   `git submodule init; git submodule update`
+2. Actual build:
+   `docker run --rm -v /tmp/syncthing-android:/mnt syncthing-android-builder ./gradlew buildNative assembleDebug`
+3. Retrieve APKs from ./app/build/outputs

docker/prebuild.sh

@@ -1,9 +1,12 @@
 #!/bin/bash -e
 
-[ -z "$SYNCTHING_ANDROID_PREBUILT" ] && echo "Prebuild disabled" && exit 0
+if [ -z "$SYNCTHING_ANDROID_PREBUILT" ]; then
+    echo "Prebuild disabled"
+    rm -rf syncthing-android
+    exit 0
+fi
 
 echo "Prepopulating gradle and go build/pkg cache"
-git clone --recurse-submodules https://github.com/syncthing/syncthing-android
 cd syncthing-android
 ./gradlew --no-daemon lint buildNative
 cd ..