Set GOTOOLCHAIN=auto in docker build containers

[silverwind]

In docker-library/golang#472 it was decided to set GOTOOLCHAIN=local in these containers which inhibits go from downloading newer toolchains when the current one is older than the one defined in go.mod.

This should fix the nightly build errors from #35877.

From docs:

When using GOTOOLCHAIN=auto or GOTOOLCHAIN=+auto, the Go command downloads newer toolchains as needed

[silverwind]

BTW I restarted https://github.com/go-gitea/gitea/actions/runs/19129348180/job/54717708169 and it completed successfully now, so the time window where builds fail is pretty narrow, less than 24 hours. But given that this is not the first time this issue has come up, I think it's good to go ahead with this.

[wxiaoguang]

It seems that we don't need this change anymore?

New builds all succeed ..... no idea about the root reason behind it.

[silverwind]

I think the root cause must be some cache issue because there's only a 23 minute gap between go tag and image tag:

golang/go@f2cd93a
docker-library/golang@63f58b7

Still it's likely this will become an issue again in the future if we bump quickly enough.

[silverwind]

Maybe it's better to leave it at default, but don't complain next time a build fails because of GOTOOLCHAIN after a go version update.

[wxiaoguang]

Maybe it's better to leave it at default, but don't complain next time a build fails because of GOTOOLCHAIN after a go version update.

Agree, I think we can add a comment before the go {version} to explain the situation: just wait for a while after a new Go version gets released, then then images will be ready in a short while.

[silverwind]

Alternative: #35901