Describe dockerGroupLayers and how to disable it

Fixes sbt#1341 

Since 1.7.1 dockerLayerGrouping is deprecated. 

Rename the setting, and add the correct 'disable' statement.

Author: DieBauer

src/sphinx/formats/docker.rst

@@ -145,13 +145,13 @@ Environment Settings
   ``dockerApiVersion``
     The docker server API version. Used to leverage new docker features while maintaining backwards compatibility.
 
-  ``dockerLayerGrouping``
+  ``dockerGroupLayers``
     The function mapping files into separate layers to increase docker cache hits.
     Lower index means the file would be a part of an earlier layer.
     The main idea behind this is to COPY dependencies *.jar's first as they should change rarely.
     In separate command COPY the application *.jar's that should change more often.
     Defaults to detect whether the file name starts with ``ThisBuild / organization``.
-    To disable layers map all files to no layer using ``dockerLayerGrouping in Docker  := (_ => None)``.
+    To disable layers map all files to no layer using ``dockerGroupLayers in Docker := PartialFunction.empty``.
 
 Publishing Settings
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