It's a generally useful file for people using buildkitd. It's also
more appropriate to write it in use-buildset-registry and then
just have build-docker-image copy it.
Do the same thing with writing the cert - we don't need to know
which path on the host use-buildset-registry wrote the file to,
we can just write the content from the dir into the container,
and then it's just a consistent command to apply it.
Change-Id: Iaa485c2e8628900dccbed1f4b0773b6d1b5f7983
70db82198c