Package diffutils is missing in Docker image

The docs mention the config_diff tool:

But the package diffutils is missing.

```

[root@mf1045-nb115 bin]# ./config-diff.sh -d

Compare running configuration with pristine configuration

Running config path : ../etc
Pristine config path: ../share/etc-pristine

Configuration files only in /opt/opennms/etc
------------------------------------------------------------
./config-diff.sh: line 50: diff: command not found

Configuration files modified by the user
------------------------------------------------------------
./config-diff.sh: line 54: diff: command not found

Configuration only in /opt/opennms/share/etc-pristine
------------------------------------------------------------
./config-diff.sh: line 58: diff: command not found

[root@mf1045-nb115 bin]# find / -name "diff"
/opt/opennms/jetty-webapps/hawtio/lib/codemirror/mode/diff
```