mesos: add docker support

nixos/modules/services/misc/mesos-slave.nix

@@ -72,6 +72,7 @@ in {
       description = "Mesos Slave";
       wantedBy = [ "multi-user.target" ];
       after = [ "network-interfaces.target" ];
+      environment.MESOS_CONTAINERIZERS = "docker,mesos";
       serviceConfig = {
 	ExecStart = ''
 	  ${pkgs.mesos}/bin/mesos-slave \
@@ -80,6 +81,7 @@ in {
 	    ${optionalString cfg.withHadoop "--hadoop-home=${pkgs.hadoop}"} \
 	    --work_dir=${cfg.workDir} \
 	    --logging_level=${cfg.logLevel} \
+	    --docker=${pkgs.docker}/libexec/docker/docker \
 	    ${toString cfg.extraCmdLineOptions}
 	'';
 	PermissionsStartOnly = true;