caddy: build via caddyserver.com download URL, not local xcaddy

The xcaddy/Go compile burns ~1GB RAM this VPS can't spare (per ~/docs/caddy.md
"Custom Binary"). Pull the prebuilt L4-enabled binary from the Caddy build
server instead and swap it over the stock binary in the official image. Built
and verified: caddy v2.11.3 with layer4.handlers.proxy + proxy_protocol.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -33,8 +33,10 @@ compose, config, ACL snippet, and Caddy build, and only *reads from the tailnet*
 - **Mailbox** (`docker-compose.yml`): Stalwart in a Tailscale sidecar via
   `network_mode: service:ts-stalwart`. Binds nothing on the host. All mail
   ports listen on the tailnet only.
-- **Edge** (`caddy/`): a layer-4 TCP proxy. Pure pass-through; Stalwart owns
-  TLS. **Can run on a different machine** than the mailbox — the key idea.
+- **Edge** (`caddy/`): a layer-4 TCP proxy (Caddy + `caddy-l4`, pulled prebuilt
+  from caddyserver.com — no local `xcaddy` build, per `~/docs/caddy.md`). Pure
+  pass-through; Stalwart owns TLS. **Can run on a different machine** than the
+  mailbox — the key idea.
 - **Backends**: data+fts → Postgres, blob → Garage S3, lookup/in-memory →
   Redis. One stalwart role/db, one Garage bucket, one Redis logical DB.
 

README.md

@@ -17,7 +17,7 @@ tailwart/
 ├── docker-compose.yml      # the mailbox: ts-stalwart sidecar + stalwart
 ├── config/config.toml      # Stalwart config — PG + Redis + S3 wiring (strawman)
 ├── caddy/                  # the edge: custom Caddy (caddy-l4) layer-4 mail proxy
-│   ├── Dockerfile          #   xcaddy build with caddy-l4 + ratelimit
+│   ├── Dockerfile          #   pulls prebuilt caddy-l4 binary (caddyserver.com, no local build)
 │   ├── caddy.json          #   :25/465/587/143/993 → stalwart over the tailnet
 │   ├── docker-compose.yml  #   deploy on any public-IP, tailnet, tag:reverse-proxy host
 │   └── README.md

caddy/Dockerfile

@@ -1,15 +1,21 @@
-# Custom Caddy with the layer-4 (TCP/UDP) app so it can proxy raw mail ports,
-# not just HTTP. caddy-ratelimit is included to match the house build on `box`.
+# Caddy with the layer-4 (TCP/UDP) app so it can proxy raw mail ports.
+#
+# Built the house way (see ~/docs/caddy.md "Custom Binary"): grab the prebuilt
+# static binary from caddyserver.com's build server, NOT a local xcaddy/Go
+# build. The compile burns ~1GB RAM, which this VPS can't spare — the download
+# server does it for us. The base image only contributes its entrypoint + CA
+# certs; we swap in the L4-enabled binary over the stock one.
 #
 #   docker build -t tailwart-caddy ./caddy
 #
-# Pinned to 2.11 to match box's Caddy. Bump deliberately.
-FROM caddy:2.11-builder AS build
-RUN xcaddy build \
-    --with github.com/mholt/caddy-l4 \
-    --with github.com/mholt/caddy-ratelimit
-
+# Add more plugins by appending &p=<url-encoded module path> to CADDY_DOWNLOAD,
+# e.g. ...&p=github.com%2Fmholt%2Fcaddy-ratelimit (the main box build has that).
 FROM caddy:2.11
-COPY --from=build /usr/bin/caddy /usr/bin/caddy
-# Proof the L4 module is in the binary (fails the build if not):
-RUN caddy list-modules | grep -q 'layer4' || (echo 'caddy-l4 missing!' && exit 1)
+
+ARG CADDY_DOWNLOAD="https://caddyserver.com/api/download?os=linux&arch=amd64&p=github.com%2Fmholt%2Fcaddy-l4"
+
+RUN apk add --no-cache curl \
+ && curl -fsSL -o /usr/bin/caddy "$CADDY_DOWNLOAD" \
+ && chmod 0755 /usr/bin/caddy
+# Fail the build loudly if the L4 module isn't actually in the binary.
+RUN caddy list-modules | grep -q 'layer4' || { echo 'caddy-l4 missing from binary!'; exit 1; }

caddy/README.md

@@ -17,11 +17,15 @@ proxying nginx/Caddy can do for *any* TCP — it just usually isn't used for it.
 ## Build & run
 
 ```bash
-docker compose up -d --build      # builds Dockerfile (xcaddy + caddy-l4), runs it
-caddy list-modules | grep layer4  # (inside the image) proof the module loaded
+docker compose up -d --build      # builds the image, runs it
 ```
 
-The build fails loudly if `caddy-l4` isn't in the resulting binary.
+The Dockerfile doesn't compile Caddy — it pulls the prebuilt L4-enabled binary
+from `caddyserver.com/api/download` (the house method, see `~/docs/caddy.md`
+"Custom Binary"), dodging the ~1GB-RAM local `xcaddy` build this VPS can't
+afford. The build still fails loudly if `caddy-l4` isn't in the downloaded
+binary. To add plugins, append `&p=<url-encoded module path>` to
+`CADDY_DOWNLOAD` in the Dockerfile.
 
 ## Edit the upstream