dockerTools: Always set imageTag attribute

The image tag can be specified or generated from the output hash.
Previously, a generated tag could be recovered from the evaluated
image with some string operations.

However, with the introduction of streamLayeredImage, it's not
feasible to compute the generated tag yourself.

With this change, the imageTag attribute is set unconditionally,
for the buildImage, buildLayeredImage, streamLayeredImage functions.

pkgs/build-support/docker/examples.nix

@@ -364,4 +364,22 @@ rec {
     created = "now";
   };
 
+  # buildImage without explicit tag
+  bashNoTag = pkgs.dockerTools.buildImage {
+    name = "bash-no-tag";
+    contents = pkgs.bashInteractive;
+  };
+
+  # buildLayeredImage without explicit tag
+  bashNoTagLayered = pkgs.dockerTools.buildLayeredImage {
+    name = "bash-no-tag-layered";
+    contents = pkgs.bashInteractive;
+  };
+
+  # buildImage without explicit tag
+  bashNoTagStreamLayered = pkgs.dockerTools.streamLayeredImage {
+    name = "bash-no-tag-stream-layered";
+    contents = pkgs.bashInteractive;
+  };
+
 }