Add -Ddocker.tag to documentation

Author: ambs

src/main/xar-resources/data/exist-building/exist-building.xml

@@ -209,6 +209,14 @@
               <para>Enable buiding eXist-db Docker images.</para>
             </entry>
           </row>
+          <row>
+            <entry>
+              <para><code>-Ddocker.tag=tag-name</code></para>
+            </entry>
+            <entry>
+              <para>Tags the built eXist-db Docker image with a specific tagname.</para>
+            </entry>
+          </row>
           <row>
             <entry>
               <para><code>-Dmac.signing=true</code></para>