build: follow-up prev: only use host userid for local dev builds

reproducibility probably needs a hardcoded userid

Also, move the UID arg later in the dockerfiles, for better caching.
(if local dev build and repro build set different UIDs, the build caches
will diverge at that step)

contrib/android/build.sh

@@ -37,10 +37,13 @@ if [ ! -z "$ELECBUILD_NOCACHE" ] ; then
     DOCKER_BUILD_FLAGS="--pull --no-cache"
 fi
 
+if [ -z "$ELECBUILD_COMMIT" ] ; then  # local dev build
+    DOCKER_BUILD_FLAGS="$DOCKER_BUILD_FLAGS --build-arg UID=$BUILD_UID"
+fi
+
 info "building docker image."
 docker build \
     $DOCKER_BUILD_FLAGS \
-    --build-arg UID=$BUILD_UID \
     -t electrum-android-builder-img \
     --file "$CONTRIB_ANDROID/Dockerfile" \
     "$PROJECT_ROOT"