Docker image size reduction

[ccosby]

• Install ppa:git-core/ppa in build stage, copy apt lists and gpg keys to main stage
• Copy docker client and plugins from upstream docker:cli image
• Perform more thorough cleanup of apt lists and cache

This reduces the overall on-disk image size by ~348MB (amd64) and the download size by ~146MB. For those of us who use runners in containers/k8s, the network traffic is reduced by a considerable 29%.