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/Dockerfile

@@ -11,11 +11,20 @@ RUN apt-get update && apt-get install -y \
 ARG PROTOC_GEN_VERSION
 ARG GRPC_GATEWAY_VERSION
 
+ENV FALAFEL_VERSION="v0.7.1"
+ENV GOCACHE=/tmp/build/.cache
+ENV GOMODCACHE=/tmp/build/.modcache
+
 RUN cd /tmp \
+  && mkdir -p /tmp/build/.cache \
+  && mkdir -p /tmp/build/.modcache \
   && export GO111MODULE=on \
   && go get github.com/golang/protobuf/protoc-gen-go@${PROTOC_GEN_VERSION} \
   && go get github.com/grpc-ecosystem/grpc-gateway/protoc-gen-grpc-gateway@${GRPC_GATEWAY_VERSION} \
-  && go get github.com/grpc-ecosystem/grpc-gateway/protoc-gen-swagger@${GRPC_GATEWAY_VERSION}
+  && go get github.com/grpc-ecosystem/grpc-gateway/protoc-gen-swagger@${GRPC_GATEWAY_VERSION} \
+  && go get github.com/lightninglabs/falafel@${FALAFEL_VERSION} \
+  && go get golang.org/x/tools/cmd/goimports \
+  && chmod -R 777 /tmp/build/
 
 WORKDIR /build