Update base image for CI

Updates base image for CI to align with gitlab-ce:

• Ruby from 2.5.3 to 2.6.3 (gitlab-org/gitlab-foss!28117 (merged), gitlab-org/gitlab-foss#57323 (closed))
• Git from 2.18 to 2.22 (gitlab-org/gitlab-foss!31578 (merged), gitlab-org/gitaly#1715 (closed))
• Chrome from 71.0 to 73.0 (gitlab-org/gitlab-foss!27762 (merged))
• Node.js from 8.x to 12.x (gitlab-org/gitlab-foss!29703 (merged))
• Yarn from 1.12 to 1.16 (gitlab-org/gitlab-foss!29703 (merged))
• GraphicsMagick from 1.3.29 to 1.3.33 (gitlab-org&1710)

Part of gitlab-org&1710