dockerTools: support meta attribute

I want to be able to set descriptions on my docker image derivations
since these can be ingested by various tooling for helpfulness.

doc/build-helpers/images/dockertools.section.md

@@ -202,6 +202,10 @@ Similarly, if you encounter errors similar to `Error_Protocol ("certificate has
 
   _Default value:_ `false`.
 
+`meta` (Attribute Set)
+
+: The `meta` attribute of the resulting derivation, as in `stdenv.mkDerivation`. Accepts `description`, `maintainers` and any other `meta` attributes.
+
 `contents` **DEPRECATED**
 
 : This attribute is deprecated, and users are encouraged to use `copyToRoot` instead.
@@ -635,6 +639,10 @@ This allows the function to produce reproducible images.
 
   _Default value:_ `false`.
 
+`meta` (Attribute Set)
+
+: The `meta` attribute of the resulting derivation, as in `stdenv.mkDerivation`. Accepts `description`, `maintainers` and any other `meta` attributes.
+
 `passthru` (Attribute Set; _optional_)
 
 : Use this to pass any attributes as [`passthru`](#chap-passthru) for the resulting derivation.

pkgs/build-support/docker/default.nix

@@ -579,10 +579,16 @@ rec {
     {
       name,
       compressor ? "gz",
+      meta ? { },
       ...
     }@args:
     let
-      stream = streamLayeredImage (removeAttrs args [ "compressor" ]);
+      stream = streamLayeredImage (
+        removeAttrs args [
+          "compressor"
+          "meta"
+        ]
+      );
       compress = compressorForImage compressor name;
     in
     runCommand "${baseNameOf name}.tar${compress.ext}" {
@@ -592,6 +598,7 @@ rec {
         inherit stream;
       };
       nativeBuildInputs = compress.nativeInputs;
+      inherit meta;
     } "${stream} | ${compress.compress} > $out"
   );
 
@@ -641,6 +648,8 @@ rec {
       includeNixDB ? false,
       # Deprecated.
       contents ? null,
+      # Meta options to set on the resulting derivation.
+      meta ? { },
     }:
 
     let
@@ -735,6 +744,7 @@ rec {
                 lib.head (
                   lib.strings.splitString "-" (baseNameOf (builtins.unsafeDiscardStringContext result.outPath))
                 );
+            inherit meta;
           }
           ''
             ${lib.optionalString (tag == null) ''
@@ -1008,6 +1018,7 @@ rec {
       includeStorePaths ? true,
       includeNixDB ? false,
       passthru ? { },
+      meta ? { },
       # Pipeline used to produce docker layers. If not set, popularity contest
       # algorithm is used. If set, maxLayers is ignored as the author of the
       # pipeline can use one of the available functions (like "limit_layers")
@@ -1233,6 +1244,7 @@ rec {
               isExe = true;
             };
             nativeBuildInputs = [ makeWrapper ];
+            inherit meta;
           }
           ''
             makeWrapper $streamScript $out --add-flags $conf

pkgs/build-support/docker/examples.nix

@@ -77,6 +77,8 @@ let
           "${nginxPort}/tcp" = { };
         };
       };
+
+      meta.description = "Basic nginx docker image example";
     };
 
 in
@@ -91,6 +93,8 @@ rec {
       paths = [ pkgs.bashInteractive ];
       pathsToLink = [ "/bin" ];
     };
+
+    meta.description = "Basic example image";
   };
 
   # 2. service example, layered on another image
@@ -138,6 +142,8 @@ rec {
         Retries = 3;
       };
     };
+
+    meta.description = "Service example, layered on another image";
   };
 
   # 3. another service example
@@ -204,6 +210,8 @@ rec {
         "USER=nobody"
       ];
     };
+
+    meta.description = "nix example to play with the container nix store";
   };
 
   # 7. example of adding something on top of an image pull by our