The doc describes a process to let the user specify a custom maven descriptor (settings.xml).  The information appears dated.

In the doc, the user mounts the file to /project/.m2/settings.xml:

che.eclipse.org/mount-path: {prod-home}/.m2/

But in CRW 2.5, that's not where the settings.xml file is placed.  (It seems we now use scl to enable maven 3.5 as seen in the command below):
scl enable rh-maven35 'mvn clean install'
 

Please guide the user to put the settings.xml file at a location of their choosing (i.e. /projects/custom-maven/settings.xml) and then update the devfile to point to it:
"scl enable rh-maven35 'mvn --settings /projects/ustom-maven/settings.xml clean install'

Scoping:

The content is downstream specific, you can find it there: https://gitlab.cee.redhat.com/red-hat-developers-documentation/red-hat-devtools/-/blob/master/docs/topics/crw/end-user-guide/examples/crw-ref_a-kubernetes-secret-as-a-file.adoc

The documentation has been refreshed upstream. Can you verify this downstream specific example has also been refreshed, and fix it if necessary?