Build multi-platform image for linux/amd64 and linux/arm64

The builder stage cross-compiles Caddy for the target platform, so no
emulation is needed. The module check runs against the amd64 image; PR
checks also build arm64.

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

README.md

@@ -4,7 +4,7 @@ Stock [Caddy](https://caddyserver.com) plus the
 [caddy-dns/desec](https://github.com/caddy-dns/desec) module, so Caddy can
 get certificates by ACME DNS-01 against a zone hosted at
 [deSEC](https://desec.io). Published as
-`code.aneur.in/cloud/caddy-desec-docker:latest` (linux/amd64).
+`code.aneur.in/cloud/caddy-desec-docker:latest` (linux/amd64, linux/arm64).
 
 Used by the `llm` stack in [cloud/cloud](https://code.aneur.in/cloud/cloud)
 for `*.llm.aneur.in`: