* cmd/containerboot: optionally configure tailscaled with a configfile.
If EXPERIMENTAL_TS_CONFIGFILE_PATH env var is set,
only run tailscaled with the provided config file.
Do not run 'tailscale up' or 'tailscale set'.
* cmd/containerboot: store containerboot accept_dns val in bool pointer
So that we can distinguish between the value being set to
false explicitly bs being unset.
Signed-off-by: Irbe Krumina <irbe@tailscale.com>