Create public folder relative to GITEA_CUSTOM in Docker setup

[Sainan]

It may not be apparent where a custom robots.txt or such should go in the absence of a 'custom' or 'public' folder.

[silverwind]

Ok, I guess a empty folder won't hurt.

[lunny]

It seems a unnecessary change and will add two layers for image.

[Sainan]

This isn't in the Dockerfile so it will add 0 layers

[TheFox0x7]

I'd argue that it's less needed in rootless as custom directory is already made in script but overall consistency is probably better.

I think this might be good to add to container docs - that root variant has GITEA_CUSTOM predefined to /data/gitea/ and rootless does not and so it's /var/lib/gitea/custom/ otherwise someone is bound to get confused by this later on.
Alternatively... I wonder if adding a migration in the setup to move it to a proper location is worth it...? GITEA_CUSTOM there feels like it should be GITEA_WORK_DIR instead.

[wxiaoguang]

Sorry but I don't think we should add more hacky patches to the container images.

The "public" folder is a general requirement for all users, but not only Docker users.

So it should be resolved from code level.