Bump dockerfile-maven-plugin from 1.4.8 to 1.4.9

Bumps [dockerfile-maven-plugin](https://github.com/spotify/dockerfile-maven) from 1.4.8 to 1.4.9.
- [Release notes](https://github.com/spotify/dockerfile-maven/releases)
- [Changelog](https://github.com/spotify/dockerfile-maven/blob/master/CHANGELOG.md)
- [Commits](spotify/dockerfile-maven@v1.4.8...v1.4.9)

Signed-off-by: dependabot[bot] <support@dependabot.com>

pom.xml

@@ -19,7 +19,7 @@
     <easymock.version>4.0.1</easymock.version>
     <!-- Build -->
     <coveralls.version>4.3.0</coveralls.version>
-    <dockerfile-maven-plugin.version>1.4.8</dockerfile-maven-plugin.version>
+    <dockerfile-maven-plugin.version>1.4.9</dockerfile-maven-plugin.version>
     <github-release-plugin.version>1.3.0</github-release-plugin.version>
   </properties>