README: update caddy version in example commands

Also qualify caddy docker image with "docker.io/" so that the command
also works with podman.

Also update golangci-lint in hopes of fixing whatever is breaking in the
action.

Updates #cleanup

Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>