feat(build): Build Go Docker images with Go 1.12.7
Go 1.11 has issues with delve and plugins. Go 1.12 fixes this, some module stuff will need this newer version, and it's a good idea to use current versions, anyway.
https://github.com/golang/go/issues/23733#issuecomment-425673233
Tested with a 1.12 build: delve is now happy with che/axe.
Fixes canopsis#1855.