In order to have a reproducible code generation process, we need to pin
the versions of the tools used. This is accomplished easiest by
generating inside a container.
This commit adds a container image definition with fixed dependencies
for Golang proto/ttrpc code generation, and changes the agent Makefile
to invoke the update-generated-proto.sh script from within that
container.
Signed-off-by: Markus Rudy <mr@edgeless.systems>
