Revert golang.org/x/net bump

[tuxpeople]

The Dockerfile currently uses go get golang.org/x/net@v0.38.0 during build to
work around a vulnerability in the upstream Podsync repository.

Once mxpv/podsync#692 is merged and included in an
official Podsync release, this manual step should be removed.

Steps:

1. Delete the go get line and its accompanying comment from the Dockerfile.
2. Build the image and verify that the vulnerability is resolved upstream.
3. Commit the change and remove this issue.