Bump golang version to 1.12.9
Bump golang version to 1.12.9

[#167925435](https://www.pivotaltracker.com/story/show/167925435)

Co-authored-by: Maria Shaldibina <dc03896a0185453e5852a5e0f0fab6cbd2e026ff@pivotal.io>

cloudfoundry-incubator/diego-dockerfiles,cloudfoundry-incubator/diego-dockerfiles