Bound what a request can make the server hold

Cap JSON bodies at 64 KB on every API route except JMAP and uploads, which
bound themselves. Sign-in used to read a body of any size before its rate
limits ran; the flood ceiling now also runs before the body is read.

For sessions whose JMAP requests are checked, lower the read cap from 16 MB
to 4 MB, allow four such reads per session at once, and turn requests away
with a 503 once 32 MB is held across everyone.

Count sign-in limits per /64 for IPv6, since one host holds a whole /64.

Bind the compose example to loopback, and run it read-only with no
capabilities and no-new-privileges. Keep .env.* out of git and the image
build context.

docker-compose.yml

@@ -9,8 +9,21 @@ services:
         BASE_PATH: ${BASE_PATH:-}
     image: ihasmail:2
     restart: unless-stopped
+    # Loopback only: ihasmail expects a TLS reverse proxy in front of it. On
+    # every interface the app is reachable over plain HTTP, passwords and all,
+    # and with TRUST_PROXY any machine on a private network can set its own
+    # X-Forwarded-For. A proxy running in Docker can reach the service by name
+    # on the compose network and needs no published port at all.
     ports:
-      - "8080:8080"
+      - "127.0.0.1:8080:8080"
+    # The app needs no privileges and writes only to /data and /tmp.
+    read_only: true
+    tmpfs:
+      - /tmp
+    cap_drop:
+      - ALL
+    security_opt:
+      - no-new-privileges:true
     environment:
       STALWART_URL: ${STALWART_URL:?set STALWART_URL in .env}
       APP_SECRET: ${APP_SECRET:?set APP_SECRET in .env (openssl rand -base64 48)}