make+lnrpc: compile mobile RPC with docker too

Now that we have a base docker image that has all our RPC compilation
dependencies installed, we can also run the mobile RPC compilation
there. This removes the need to install falafel and goimports on the
local machine.

lnrpc/gen_protos_docker.sh

@@ -19,5 +19,6 @@ docker run \
   --rm \
   --user "$UID:$(id -g)" \
   -e UID=$UID \
+  -e COMPILE_MOBILE \
   -v "$DIR/../:/build" \
   lnd-protobuf-builder