dockerTools.buildImage: support impure dates

Because dates are an impurity, by default buildImage will use a static
date of one second past the UNIX Epoch. This can be a bit frustrating
when listing docker images in the CLI:

    $ docker image list
    REPOSITORY   TAG      IMAGE ID       CREATED        SIZE
    hello        latest   08c791c7846e   48 years ago   25.2MB

If you want to trade the purity for a better user experience, you can
set created to now.

    pkgs.dockerTools.buildImage {
      name = "hello";
      tag = "latest";
      created = "now";
      contents = pkgs.hello;

      config.Cmd = [ "/bin/hello" ];
    }

and now the Docker CLI will display a reasonable date and sort the
images as expected:

    $ docker image list
    REPOSITORY   TAG      IMAGE ID       CREATED              SIZE
    hello        latest   de2bf4786de6   About a minute ago   25.2MB

1 files changed, 39 insertions, 0 deletions

diff --git a/doc/functions.xml b/doc/functions.xml
index 3cfc6884bd2..31b40fb084a 100644
--- a/doc/functions.xml
+++ b/doc/functions.xml
@@ -638,6 +638,45 @@ buildImage {
      <literal>pkgs.cacert</literal> to <varname>contents</varname>.
     </para>
    </note>
+
+   <example xml:id="example-pkgs-dockerTools-buildImage-creation-date">
+     <title>Impurely Defining a Docker Layer's Creation Date</title>
+     <para>
+       Because dates are an impurity, by default
+       <function>buildImage</function> will use a static date of one
+       second past the UNIX Epoch. This can be a bit frustrating when
+       listing docker images in the CLI:
+     </para>
+     <screen><![CDATA[
+$ docker image list
+REPOSITORY   TAG      IMAGE ID       CREATED        SIZE
+hello        latest   08c791c7846e   48 years ago   25.2MB
+]]></screen>
+     <para>
+       If you want to trade the purity for a better user experience,
+       you can set <literal>created</literal> to
+       <literal>now</literal>.
+     </para>
+     <programlisting><![CDATA[
+pkgs.dockerTools.buildImage {
+  name = "hello";
+  tag = "latest";
+  created = "now";
+  contents = pkgs.hello;
+
+  config.Cmd = [ "/bin/hello" ];
+}
+]]></programlisting>
+     <para>
+       and now the Docker CLI will display a reasonable date and
+       sort the images as expected:
+     </para>
+     <screen><![CDATA[
+$ docker image list
+REPOSITORY   TAG      IMAGE ID       CREATED              SIZE
+hello        latest   de2bf4786de6   About a minute ago   25.2MB
+]]></screen>
+   </example>
   </section>
 
   <section xml:id="ssec-pkgs-dockerTools-fetchFromRegistry">