Upgrade Golang versions in custom Docker build
- go1.13 -> go1.13.4
- go1.12.5 -> go1.12.13
The 1.12 upgrade matches what we are using for Omnibus: gitlab-omnibus-builder!125 (merged)
The 1.12 upgrade matches what we are using for Omnibus: gitlab-omnibus-builder!125 (merged)