containers/rabbitmq: fix startup by adding a shell

erlang tries to shell out to find one of its own(!) executables, so we add a
minimal dash.

containers/rabbitmq.nix

@@ -37,6 +37,10 @@ in {
                   installTargets = ["install"];
                 };
               };
+              sh-minimal = pkgs.runCommand "sh-minimal" {} ''
+                mkdir -p $out/bin
+                cp ${pkgs.dash}/bin/dash $out/bin/sh
+              '';
 
               userName = "rabbitmq";
 
@@ -51,6 +55,7 @@ in {
                   # 11M that can probably be avoided
                   pkgs.coreutils
                   pkgs.gnused
+                  sh-minimal
                 ];
 
                 config = {