Pin version of protoc-gen-go

Signed-off-by: Andrew Mason <amason@slack-corp.com>

docker/proto/Dockerfile

@@ -1,21 +1,30 @@
 FROM golang:1.14-buster
 
-ARG PROTOC_VERSION=3.12.4
-
 # Install depedencies
 RUN apt-get update && \
  DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
  build-essential \
  ca-certificates \
  curl \
  unzip \
- wget && \
- go get \
- golang.org/x/tools/cmd/goimports \
- github.com/golang/protobuf/protoc-gen-go && \
- go install \
+ wget
+
+ARG PROTOC_GEN_GO_VERSION=v1.3.2
+
+# Need to set up a module in order to install a specific version of protoc-gen-go.
+# 1. The `go get package@version` syntax is only supported in module mode.
+# 2. In module mode, `go get` is unsupported outside of a module.
+# These two things together force us to do this quick hack.
+#
+# See https://github.com/golang/go/issues/24250 as just one of many issues on the subject.
+RUN mkdir proto-builder && \
+ cd proto-builder && \
+ go mod init proto-builder && \
+ GO111MODULE=on go get \
  golang.org/x/tools/cmd/goimports \
- github.com/golang/protobuf/protoc-gen-go
+ github.com/golang/protobuf/protoc-gen-go@${PROTOC_GEN_GO_VERSION}
+
+ARG PROTOC_VERSION=3.12.4
 
 # Download protoc
 RUN curl -LO https://github.com/protocolbuffers/protobuf/releases/download/v${PROTOC_VERSION}/protoc-${PROTOC_VERSION}-linux-x86_64.zip && \