Repro case:
$ make clean generated_files
$ hack/update-generated-protobuf.sh

This would complain about not finding `fmt`, and it was indicating the wrong
GOROOT.  The problem was that the first step built binaries for generating
code, which *embeds* the value of GOROOT into the binary.  The whole tree was
bind-mounted into the build container and then JUST the dockerized dir was
mounted over it.  The in-container build tried to use the existing binaries,
but GOROOT is wrong.

This change whites-out the whole _output dir.

I first made just an anonymous volume for _output, but docker makes that as
root, which means I can't write to it from our non-root build.  So I just put
it in the data container.  This seems to work.  The biggest change this makes
is that the $GOPATH/bin/ and $GOPATH/pkg/ dirs will persist across dockerized
builds.