feat(dnsmasq): upgrade to Debian forky and add --conf-poll patch

Bump the dnsmasq container to debian:forky-20260623-slim (dnsmasq
2.93-1), which is close enough to our dev tree that both vendored
patches now apply cleanly, and rebase dhcp-allowed-srvids.patch onto
that version.

Add conf-poll.patch, a new dnsmasq feature (--conf-poll /
--conf-poll-interval=<seconds>, default 1s) written to solve a real
problem in our Ironic setup: dnsmasq relies on inotify to notice
changes to --hostsdir, --dhcp-hostsdir and --dhcp-optsdir. When those
directories live on an NFS share and are modified or created by a
different NFS client (i.e. not the machine running dnsmasq itself),
the local kernel never gets an inotify event for it, so dnsmasq
silently keeps serving stale DNS/DHCP data until it's sent a SIGHUP or
restarted.

--conf-poll makes dnsmasq additionally stat() those directories on a
timer (every --conf-poll-interval seconds) and reload anything that's
new, changed, or removed, using the exact same reload logic as an
inotify event or SIGHUP would trigger. It runs alongside inotify, not
instead of it, so on non-NFS setups where inotify works fine it's just
a harmless no-op backup check. It's opt-in and disabled by default, so
existing deployments are unaffected unless the option is explicitly
set. Deleting a file from --dhcp-hostsdir/--dhcp-optsdir intentionally
still does not remove the DHCP config it added, since that already
matches how those two options have always behaved.

Author: skrobul

containers/dnsmasq/Dockerfile

@@ -1,11 +1,11 @@
 # syntax=docker/dockerfile:1
-FROM debian:bookworm-20240408-slim AS builder
+FROM debian:forky-20260623-slim AS builder
 
 ENV DEBFULLNAME="Marek Skrobacki"
 ENV DEBEMAIL="marek.skrobacki@rackspace.co.uk"
 ENV QUILT_PATCHES=debian/patches
 
-RUN echo "deb-src http://deb.debian.org/debian bookworm main" > /etc/apt/sources.list
+RUN echo "deb-src http://deb.debian.org/debian forky main" > /etc/apt/sources.list
 RUN apt-get update && apt-get -y install \
     build-essential \
     devscripts \
@@ -21,21 +21,24 @@ RUN apt-get update && apt-get -y install \
 WORKDIR /src
 RUN apt-get -y build-dep dnsmasq
 RUN apt-get -y source dnsmasq
-# copy in our patch
+# copy in our patches
 COPY containers/dnsmasq/dnsmasq/dhcp-allowed-srvids.patch /src/
-# setup the patch to be built into the quilt file
+COPY containers/dnsmasq/dnsmasq/conf-poll.patch /src/
+# setup the patches to be built into the quilt file
 # set our version number to a local override suffixed by '.uc1'
 RUN cd /src/dnsmasq-* && \
     mkdir -p debian/patches && \
     mv /src/dhcp-allowed-srvids.patch debian/patches && \
-    dch -l .uc "patched for dhcp-allowed-srvids" && \
+    mv /src/conf-poll.patch debian/patches && \
+    dch -l .uc "patched for dhcp-allowed-srvids and conf-poll" && \
     echo dhcp-allowed-srvids.patch >> debian/patches/series && \
+    echo conf-poll.patch >> debian/patches/series && \
     quilt push -a
 # build it
 RUN cd /src/dnsmasq-* && dpkg-buildpackage -rfakeroot
 
 
-FROM debian:bookworm-20240408-slim AS prod
+FROM debian:forky-20260623-slim AS prod
 
 LABEL org.opencontainers.image.description="dnsmasq for Understack's Ironic"