diff --git a/system/machine/RPi5/docker.nix b/system/machine/RPi5/docker.nix
new file mode 100644
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--- /dev/null
+++ b/system/machine/RPi5/docker.nix
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+# Docker host with the daemon socket exposed over the network.
+#
+# SECURITY: the daemon listens on plain TCP 2375 with NO TLS and NO auth. Access
+# to that port is root-equivalent on this host (the Docker API can mount the
+# host filesystem and run privileged containers). The ONLY thing protecting it
+# is the nftables rule below, which accepts 2375 solely from the trusted LAN
+# subnet. Do not widen that subnet to anything you do not fully trust. The
+# secure upgrade path is mutual TLS on 2376 (--tlsverify with client certs);
+# that needs out-of-band cert provisioning and is intentionally not wired here.
+{ ... }:
+{
+  virtualisation.docker.enable = true;
+
+  # Expose the daemon over TCP by extending systemd socket activation rather than
+  # setting daemon.settings.hosts. The NixOS docker unit starts dockerd with
+  # `-H fd://` and takes its listeners from this socket; putting `hosts` in
+  # daemon.json as well would conflict with that and dockerd would refuse to
+  # start. Adding the TCP listener here keeps a single source of truth.
+  # The leading "" resets the unit's default (unix-socket-only) ListenStream list.
+  systemd.sockets.docker.socketConfig.ListenStream = [
+    ""
+    "/run/docker.sock"
+    "0.0.0.0:2375"
+  ];
+
+  # Source-restricted firewall rule for the Docker TCP port. 2375 is deliberately
+  # NOT added to networking.firewall.allowedTCPPorts (that would open it to every
+  # source); instead nftables accepts it only from the trusted subnet. Adjust the
+  # CIDR to match the LAN that should reach the Docker API.
+  networking.nftables.enable = true;
+  networking.firewall.extraInputRules = ''
+    ip saddr 10.187.1.0/24 tcp dport 2375 accept
+  '';
+}