IPCR Forge: public front door (docker pull without IPCR)

ipcr-public: a second ipcrd behind Caddy at ipcr-<domain>, plain HTTP, no
pinning, off the admin and CI networks. With PUBLIC_POLICY it serves only
this forge's publisher name, its verified ENS/DNS name and its published
image roots; everything else is 404, pushes 405. So

  docker pull ipcr-<domain>/ipns/metadec.eth/ipcr-hello:1.0.0

works on any machine with plain Docker. Switch in the admin page
(config.json "public"), mirrored into published.json for the pages, which
show the public pull lines while it is on.

ipcr 1.5.0, bridge 0.4.0. Verified from a second machine (docs/forge.md).

Apps/IPCR-Forge/rationale.md

@@ -48,6 +48,8 @@ with. See docs/forge.md.
     bridge.
 15. **The root organisation** (`IPCR_ROOT_ORG`): setup creates it if missing, and its repositories
     are also published at the root of the forge's name.
+16. **A public, unauthenticated, read-only registry route** (`ipcr-<domain>` → `ipcr-public`), so this
+    forge's images can be pulled without IPCR.
 
 ## Why it is necessary
 
@@ -132,6 +134,11 @@ already mounts read-only.
 repositories may use them. A Gitea organisation makes "who may publish there" a matter of team
 membership, managed in Gitea.
 
+**16. The public front door.** The IPFS address only works on machines running IPCR. People trying
+an image elsewhere (a laptop, a CI on another platform) need a registry that plain Docker can pull
+from. Public IPFS gateways cannot do it, since they have no registry API. A login would defeat the
+purpose: the images are public on IPFS anyway.
+
 ## Security mitigations in place
 
 **The bridge**
@@ -182,6 +189,18 @@ membership, managed in Gitea.
   and its secret at once. "Reissue" rotates a token.
 - **Gate container:** `$PUID`, no capabilities, read-only root filesystem, 64 MB.
 
+**The public front door**
+- **This forge's images only.** It answers only for the publisher's own IPNS name, its verified
+  ENS/DNS name, and the root CIDs of published images. Anything else is 404 before any resolution,
+  so strangers cannot use it to fetch or serve other IPFS content.
+- **Read-only and storage-neutral.** GET and HEAD only, and `AUTO_PIN=false`: a pull never pins.
+- **Isolated.**
+  - It is a separate process (`ipcr-public`) on `pcs` and `ipcr-internal` only, not on the admin
+    or CI networks.
+  - It runs as `$PUID` with no capabilities and a read-only root filesystem, capped at 256 MB.
+  - It reads the main gateway's state read-only.
+- **Can be switched off** in the admin pages (`config.json` `"public": false`).
+
 **The page**
 - **The public page cannot change anything.** The bridge serves it read-only: the embedded page,
   IPCR's `published.json` (mounted read-only) and the bridge's repository list.