Add instructions for using docker to regen derived files (#3489)

This is much cleaner than the previous approach of requiring protoc to be installed globally on the machine, which inevitably causes tensions with other projects that are developed on the same machine. We may want to consider building and publishing the Docker image in a repository so that people can just pull it and use it unless they are upgrading any of the tool versions (e.g. protoc).

The changes to the `batchmap` code are the results of regenerating all files using these new instructions.

README.md

@@ -167,7 +167,21 @@ Some of the Trillian Go code is autogenerated from other files:
    golang.org/x/tools/cmd/stringer`).
 
 Re-generating mock or protobuffer files is only needed if you're changing
-the original files; if you do, you'll need to install the prerequisites:
+the original files. The recommended way to do this is by using the Docker
+image used by the Cloud Build:
+
+```shell
+docker build -f ./integration/cloudbuild/testbase/Dockerfile -t trillian-builder .
+docker run -it --mount type=bind,src="$(pwd)",target=/src trillian-builder /bin/bash -c "cd /src; ./scripts/install_deps.sh; go generate -x ./..."
+```
+
+These commands first create a docker image from the Dockerfile in this repo, and
+then launch a container based on this image with the local directory mounted. The
+correct versions of the tools are determined using the `go.mod` file in this repo,
+and these tools are installed. Finally, all of the generated files are regenerated
+and Docker exits.
+
+Alternatively, you can install the prerequisites locally:
 
   - a series of tools, using `go install` to ensure that the versions are
     compatible and tested:

scripts/install_deps.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+#
+# Installs the dependencies required to build this repo.
+set -eu
+
+main() {
+  go install github.com/golang/mock/mockgen
+  go install google.golang.org/protobuf/proto
+  go install google.golang.org/protobuf/cmd/protoc-gen-go
+  go install google.golang.org/grpc/cmd/protoc-gen-go-grpc
+  go install github.com/pseudomuto/protoc-gen-doc/cmd/protoc-gen-doc
+  go install golang.org/x/tools/cmd/stringer
+}
+
+main