new-app: configure jail mail relay and persist the jail's network alias

Two provisioning gaps in new-app:

- The jail's dma.conf inherited the base image's shared SMARTHOST/
  PORT/AUTHPATH, but never got a per-app MAILNAME - without one, dma
  can misroute mail for recipients on the jail's own apex domain as
  local delivery. EnableMail appends a MAILNAME of <app>.skiff.local,
  which can't collide with any real domain an app sends to or from.

- A classic jail's ip4.addr is only a bind restriction, not a route,
  until something aliases it onto a host interface. jail.conf.tmpl
  never set jail(8)'s own `interface` parameter, so nothing ever
  created that alias - confirmed on skiff-freebsd, where the address
  had twice been made to work via an ad hoc `ifconfig alias` that
  didn't survive a reboot. Setting `interface` lets jail(8) manage
  the alias itself, including on every boot.

2 files changed, 2 insertions, 0 deletions

diff --git a/internal/tmpl/jail.conf.tmpl b/internal/tmpl/jail.conf.tmpl
index 446c8c8..c82b3a6 100644
--- a/internal/tmpl/jail.conf.tmpl
+++ b/internal/tmpl/jail.conf.tmpl
@@ -1,6 +1,7 @@
 {{.Name}} {
 	path = "{{.JailRoot}}";
 	host.hostname = "{{.Name}}.skiff.local";
+	interface = "{{.Iface}}";
 	ip4.addr = "{{.IP}}";
 	exec.start = "/bin/sh /etc/rc";
 	exec.stop = "/bin/sh /etc/rc.shutdown";
diff --git a/internal/tmpl/tmpl.go b/internal/tmpl/tmpl.go
index 68e917d..12baa70 100644
--- a/internal/tmpl/tmpl.go
+++ b/internal/tmpl/tmpl.go
@@ -37,6 +37,7 @@ type JailConfData struct {
 	Name     string
 	JailRoot string
 	IP       string
+	Iface    string
 }
 
 func JailConf(d JailConfData) (string, error) { return Render(jailConf, d) }