docs(sidecar): make UDP MTU env-overridable end-to-end

The sidecar entrypoint hardcoded `udp.mtu: 1472`, the Docker-bridge
IPv4 maximum (1500 MTU - 8 UDP - 20 IPv4 header). Promote it to
`FIPS_UDP_MTU` (defaulting to 1472, preserving behavior) so non-Docker
reuses of the example can override without editing the script. Plumb
the env var through `docker-compose.yml` so a host-level setting
reaches the container, and add a row to the README's env-var table.

Also annotate the static-CI node template with a comment explaining
the same 1472 rationale and the daemon's 1280 default. The template
keeps 1472 as the literal value since the CI suite runs on a Docker
bridge where that's correct.

No behavior change unless the host explicitly sets FIPS_UDP_MTU.

testing/static/configs/node.template.yaml

@@ -17,6 +17,8 @@ dns:
 transports:
   udp:
     bind_addr: "0.0.0.0:2121"
+    # Docker bridge supports 1500-MTU IPv4 so 1472 is safe here.
+    # Operator-facing daemon default is 1280 (IPv6 minimum).
     mtu: 1472
     # recv_buf_size: 2097152   # 2 MB (default)
     # send_buf_size: 2097152   # 2 MB (default)