make+lnrpc: use docker to compile protos

This commit aims to make it easier for developers to compile our
protobuf definitions. They now only need to have docker installed
instead of a whole set of binaries and libraries all pinned to very
specific versions.

lnrpc/gen_protos_docker.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+
+set -e
+
+# Directory of the script file, independent of where it's called from.
+DIR="$(cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd)"
+
+PROTOC_GEN_VERSION=$(go list -f '{{.Version}}' -m github.com/golang/protobuf)
+GRPC_GATEWAY_VERSION=$(go list -f '{{.Version}}' -m github.com/grpc-ecosystem/grpc-gateway)
+
+echo "Building protobuf compiler docker image..."
+docker build -q -t lnd-protobuf-builder \
+  --build-arg PROTOC_GEN_VERSION="$PROTOC_GEN_VERSION" \
+  --build-arg GRPC_GATEWAY_VERSION="$GRPC_GATEWAY_VERSION" \
+  .
+
+echo "Compiling and formatting *.proto files..."
+docker run \
+  --rm \
+  --user "$UID:$(id -g)" \
+  -e UID=$UID \
+  -v "$DIR/../:/build" \
+  lnd-protobuf-builder