Fail loudly when CUPS cannot bind port 631

The queue was still advertised over mDNS while cupsd never bound TCP 631:
a rootless Podman container on the host network cannot bind the privileged
port. Apple clients then discovered a printer they could not reach and
reported a confusing _ipps connection failure.

* entrypoint.sh: verify that CUPS accepts connections on 631 after startup
  and exit with an actionable message otherwise
* entrypoint.sh: mark the script executable in the repository
* cupsd.conf: drop BrowseRemoteProtocols, which CUPS 2.4 no longer supports
  and which logs an error on every start
* README: add a troubleshooting section for the port-binding failure and the
  _ipps/TLS certificate requirement, and document the /etc/cups bind-mount
  caveat plus the CUPS 3 PPD deprecation

cupsd.conf

@@ -18,7 +18,6 @@ Listen /run/cups/cups.sock
 # Advertise shared printers on the local network via mDNS/DNS-SD (Avahi).
 Browsing On
 BrowseLocalProtocols dnssd
-BrowseRemoteProtocols none
 BrowseWebIF Yes
 
 # Default authentication type, when authentication is required...