also persist go pkgs on docker image

docker/Dockerfile

@@ -37,6 +37,9 @@ ENV SYNCTHING_ANDROID_PREBUILT 1
 # Set location of go cache
 ENV GOCACHE /opt/gocache
 
+# Set location of GOPATH to persist packages for module builds in GOPATH/pkg/mod
+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

docker/prebuild.sh

@@ -33,7 +33,7 @@ for ARCH in arm x86 arm64; do
       GOOS=android GOARCH=$GOARCH go install -v std
 done
 
-echo "Prepopulating gradle cache"
+echo "Prepopulating gradle and go build/pkg cache"
 git clone https://github.com/syncthing/syncthing-android
 cd syncthing-android
 ./gradlew --no-daemon lint

syncthing/build-syncthing.py

@@ -109,7 +109,6 @@ for target in BUILD_TARGETS:
 
     environ = os.environ.copy()
     environ.update({
-        'GOPATH': module_dir,
         'GO111MODULE': 'on',
         'CGO_ENABLED': '1',
     })