Use usual image base

Go bug has been fixed in 1.15, no need for libstdc++.so.6 anymore.

This works fine:

~ docker run --rm registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:578466e-race
Unable to find image 'registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:578466e-race' locally
578466e-race: Pulling from gitlab-org/cluster-integration/gitlab-agent/agentk
e8d8785a314f: Already exists
e005d777a298: Already exists
dc311b4e5c64: Pull complete
Digest: sha256:4b90767b9ee8d4afa03b27ec4adf5d31a61346aff6f4c3d219dd5a91a9a63226
Status: Downloaded newer image for registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:578466e-race
time="2020-09-08T06:50:54Z" level=error msg="Program aborted" error="token file: open : no such file or directory"