We assume most containers are immutable and don't expect tailscale
running in them to auto-update. But there's no reason to prohibit it
outright.
Ignore the tailnet-wide default auto-update setting in containers, but
allow local users to turn on auto-updates via the CLI.
RELNOTE=Auto-updates are allowed in containers, but ignore the tailnet-wide default.
Fixes#12292
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>