dockerTools: optionally preserve directory symlinks

In some cases, this seems to save a lot (>40%) of space.

(cherry picked from commit dce2c258ac33ee1cd3b3e852b36e1d8fa11f33a1)

pkgs/build-support/docker/default.nix

···
     baseJson,
     # Files to add to the layer.
     contents ? null,
+    # When copying the contents into the image, preserve symlinks to
+    # directories (see `rsync -K`).  Otherwise, transform those symlinks
+    # into directories.
+    keepContentsDirlinks ? false,
     # Additional commands to run on the layer before it is tar'd up.
     extraCommands ? "", uid ? 0, gid ? 0
   }:
···
         echo "Adding contents..."
         for item in $contents; do
           echo "Adding $item"
-          rsync -ak --chown=0:0 $item/ layer/
+          rsync -a${if keepContentsDirlinks then "K" else "k"} --chown=0:0 $item/ layer/
         done
       else
         echo "No contents to add to layer."
···
     runAsRoot,
     # Files to add to the layer. If null, an empty layer will be created.
     contents ? null,
+    # When copying the contents into the image, preserve symlinks to
+    # directories (see `rsync -K`).  Otherwise, transform those symlinks
+    # into directories.
+    keepContentsDirlinks ? false,
     # JSON containing configuration and metadata for this layer.
     baseJson,
     # Existing image onto which to append the new layer.
···
         echo "Adding contents..."
         for item in ${toString contents}; do
           echo "Adding $item..."
-          rsync -ak --chown=0:0 $item/ layer/
+          rsync -a${if keepContentsDirlinks then "K" else "k"} --chown=0:0 $item/ layer/
         done
 
         chmod ug+w layer
···
     fromImageTag ? null,
     # Files to put on the image (a nix store path or list of paths).
     contents ? null,
+    # When copying the contents into the image, preserve symlinks to
+    # directories (see `rsync -K`).  Otherwise, transform those symlinks
+    # into directories.
+    keepContentsDirlinks ? false,
     # Docker config; e.g. what command to run on the container.
     config ? null,
     # Optional bash script to run on the files prior to fixturizing the layer.
···
         if runAsRoot == null
         then mkPureLayer {
           name = baseName;
-          inherit baseJson contents extraCommands uid gid;
+          inherit baseJson contents keepContentsDirlinks extraCommands uid gid;
         } else mkRootLayer {
           name = baseName;
           inherit baseJson fromImage fromImageName fromImageTag
-                  contents runAsRoot diskSize extraCommands;
+                  contents keepContentsDirlinks runAsRoot diskSize
+                  extraCommands;
         };
       result = runCommand "docker-image-${baseName}.tar.gz" {
         buildInputs = [ jshon pigz coreutils findutils jq ];