Dockerfile: fixed issue when GIT_USER_NAME is undefined.

Author: gfraiteur

src/PostSharp.Engineering.BuildTools/Docker/EpilogueComponent.cs

@@ -49,8 +49,8 @@ ARG GITDIRS
                         } `
                     } `
                 }
-            RUN git config --global user.name $env:GIT_USER_NAME; `
-                git config --global user.email $env:GIT_USER_EMAIL;
+            RUN if ( $env:GIT_USER_NAME ) { git config --global user.name $env:GIT_USER_NAME } `
+                if ( $env:GIT_USER_EMAIL ) { git config --global user.email $env:GIT_USER_EMAIL }
             """ );
     }