Bump nexus-staging-maven-plugin in /dockerfile-image-update

Bumps nexus-staging-maven-plugin from 1.6.8 to 1.6.13.

---
updated-dependencies:
- dependency-name: org.sonatype.plugins:nexus-staging-maven-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

dockerfile-image-update/pom.xml

@@ -253,7 +253,7 @@
                     <plugin>
                         <groupId>org.sonatype.plugins</groupId>
                         <artifactId>nexus-staging-maven-plugin</artifactId>
-                        <version>1.6.8</version>
+                        <version>1.6.13</version>
                         <extensions>true</extensions>
                         <configuration>
                             <serverId>ossrh</serverId>