cmd/upspin-deploy-gcp: include mailconfig in keyserver deploy
cmd/upspin-deploy-gcp: include mailconfig in keyserver deploy

Change d7fa3b6 accidentally removed the inclusion of the mailconfig file
when preparing the keyserver Docker image. Restore that behavior.

Change-Id: I93387b7d944e0c60a8c0365e1831dcc616d3fa58
Reviewed-on: https://upspin-review.googlesource.com/13340
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>

upspin/gcp,upspin/gcp