deploy: add CAP_KILL so opendkim reload on domain add/remove works

supervisord runs as root inside the container but cap_drop: ALL still
blocked it from signaling opendkim (a different uid) — cross-uid
kill() checks CAP_KILL regardless of the caller's uid. Domain add was
failing in prod with "unknown problem sending sig opendkim ...
PermissionError: Operation not permitted".

deploy/docker-compose.yml

@@ -75,6 +75,12 @@ services:
     #   - SETUID/SETGID    — supervisord drops the panel to the unprivileged
     #                        `panel` user; Postfix switches to its own users;
     #   - DAC_OVERRIDE     — cross-user file access within the `selfpost` group.
+    #   - KILL             — supervisord (root) sends SIGUSR1 to opendkim (a
+    #                        different uid) on domain add/remove to reload its
+    #                        KeyTable/SigningTable; without it os.kill() fails
+    #                        with EPERM even though the caller is root, because
+    #                        the kernel's signal permission check for
+    #                        cross-uid kill() still consults CAP_KILL.
     # FOWNER/FSETID are required by build/entrypoint.sh's permission
     # self-healing; without them chmod fails with EPERM and the container
     # crash-loops on start.
@@ -90,3 +96,4 @@ services:
       - SETUID
       - SETGID
       - DAC_OVERRIDE
+      - KILL