dockerTools: use makeOverridable for buildImage family of functions

this allows nix users to modify existing images without having
to rely on container image inheritance mechanisms via fromImage

pkgs/build-support/docker/default.nix

@@ -487,7 +487,7 @@ rec {
       '';
     };
 
-  buildLayeredImage = { name, ... }@args:
+  buildLayeredImage = lib.makeOverridable ({ name, ... }@args:
     let
       stream = streamLayeredImage args;
     in
@@ -496,7 +496,8 @@ rec {
         inherit (stream) imageName;
         passthru = { inherit (stream) imageTag; };
         nativeBuildInputs = [ pigz ];
-      } "${stream} | pigz -nTR > $out";
+      } "${stream} | pigz -nTR > $out"
+  );
 
   # 1. extract the base image
   # 2. create the layer
@@ -504,7 +505,7 @@ rec {
   # 4. compute the layer id
   # 5. put the layer in the image
   # 6. repack the image
-  buildImage =
+  buildImage = lib.makeOverridable (
     args@{
       # Image name.
       name
@@ -751,7 +752,8 @@ rec {
       '';
 
     in
-    checked result;
+    checked result
+  );
 
   # Merge the tarballs of images built with buildImage into a single
   # tarball that contains all images. Running `docker load` on the resulting
@@ -837,7 +839,7 @@ rec {
     })
   );
 
-  streamLayeredImage =
+  streamLayeredImage = lib.makeOverridable (
     {
       # Image Name
       name
@@ -1046,7 +1048,8 @@ rec {
           makeWrapper ${streamScript} $out --add-flags ${conf}
         '';
       in
-      result;
+      result
+  );
 
   # This function streams a docker image that behaves like a nix-shell for a derivation
   streamNixShellImage =