Add documentation note that LABEL allows environment replacement

Signed-off-by: Tim Waugh <twaugh@redhat.com>

docs/reference/builder.md

@@ -155,6 +155,7 @@ the `Dockerfile`:
 * `COPY`
 * `ENV`
 * `EXPOSE`
+* `LABEL`
 * `USER`
 * `WORKDIR`
 * `VOLUME`