Docker refactoring

[mFarghaly]

PR Summary

• Setting up non-root ethereum user:group for the docker container and following best security practices
• Creating /home/ethereum home directory for the docker image instead of /root
• Updating go.mod version to 1.16
• Updating docker builder image to alpine 1.16.4
• Changing all occcuraences of /root directory in the documentation to /home/ethereum

After building and running the docker image

$ docker exec <container-id> id
uid=1000(ethereum) gid=1000(ethereum) # was uid=1(root) gid=1(root)

$ docker exec <container-id> whomi
ethereum # was root

$ docker exec <container-id> which geth
/home/ethereum/bin/geth # was /usr/bin/geth

[karalabe]

Thanks for this PR, but it's unfortunately a no go. Many people use our docker images either as standalone Geth instances or building further layers on top. The behavior at this point is considered API and we can't afford to change things. Changing permissions would most probably result in broken deploys or permission denied errors for existing users. Changing locations would result in broken higher level images or automations/mounts built on top.

Btw, the 1.16 docker images defaults to the latest minor release in the 1.16 family, so currently 1.16.4, no need to be explicit (actually, better the current way because we don't need to follow minor Go releases).

Go mod was deliberately not updated to 1.16, there were previous PRs. We don't see a reason to do so.