Build Caddy with the caddy-dns/desec module

Stock caddy:2-alpine plus caddy-dns/desec, for ACME DNS-01 against zones
hosted at deSEC (*.llm.aneur.in in cloud/cloud's llm stack). CI builds
and checks the module on pull requests, and pushes :latest (keeping
:previous) on main, weekly and by hand.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,8 @@
+# Caddy with the caddy-dns/desec module, for ACME DNS-01 challenges against
+# zones hosted at deSEC. Otherwise the stock caddy:2-alpine image.
+
+FROM caddy:2-builder AS builder
+RUN xcaddy build --with github.com/caddy-dns/desec
+
+FROM caddy:2-alpine
+COPY --from=builder /usr/bin/caddy /usr/bin/caddy