Mailcow: Override dockerapi image.

Change-Id: I62507e59e46f938a1d6d82a1a28bf21b9f8b6557
diff --git a/mailcow/default.nix b/mailcow/default.nix
index 0408755..355557a 100644
--- a/mailcow/default.nix
+++ b/mailcow/default.nix
@@ -38,6 +38,9 @@
         netfilter-mailcow:
           build: ./data/Dockerfiles/netfilter
 
+        dockerapi-mailcow:
+          build: ./data/Dockerfiles/dockerapi
+
         watchdog-mailcow:
           build: ./data/Dockerfiles/watchdog