Merge pull request #66543 from grahamc/dt

dockerTools: document extraCommands, increase layers

doc/functions/dockertools.xml

@@ -312,7 +312,23 @@ hello        latest   de2bf4786de6   About a minute ago   25.2MB
       Maximum number of layers to create.
      </para>
      <para>
-      <emphasis>Default:</emphasis> <literal>24</literal>
+      <emphasis>Default:</emphasis> <literal>100</literal>
+     </para>
+     <para>
+      <emphasis>Maximum:</emphasis> <literal>125</literal>
+     </para>
+    </listitem>
+   </varlistentry>
+   <varlistentry>
+    <term>
+     <varname>extraCommands</varname> <emphasis>optional</emphasis>
+    </term>
+    <listitem>
+     <para>
+       Shell commands to run while building the final layer, without access
+       to most of the layer contents. Changes to this layer are "on top"
+       of all the other layers, so can create additional directories
+       and files.
      </para>
     </listitem>
    </varlistentry>

pkgs/build-support/docker/default.nix

@@ -291,9 +291,10 @@ rec {
     # Files to add to the layer.
     closure,
     configJson,
-    # Docker has a 42-layer maximum, we pick 24 to ensure there is plenty
-    # of room for extension
-    maxLayers ? 24
+    # Docker has a 125-layer maximum, we pick 100 to ensure there is
+    # plenty of room for extension.
+    # https://github.com/moby/moby/blob/b3e9f7b13b0f0c414fa6253e1f17a86b2cff68b5/layer/layer_store.go#L23-L26
+    maxLayers ? 100
   }:
     let
       storePathToLayer = substituteAll