JAIL-15 Post-push build for containerized masking

[johngallagher-dlpx]

This adds a new package for a new flavor of the masking product. Although the output of this job will not be part of the appliance, we are using linux-pkg since linux-pkg provides a number of useful feature for building generic software, like automatically generated Jenkins jobs that will trigger on post-push, the creation of a build environment with the correct build dependencies installed, a simple way to upload output artifacts to S3, etc.

Because this package is not used by the appliance, we do not include it in any of the package lists, so that its build is not triggered by the build-packages job or by git ab-pre-push.

This new artifact could be built as part of the existing masking package by just adding another task to the Gradle invocation in that package's config.sh. However, since this package will be used independently, it seemed worth it to keep its build independent, even if it means spending a couple minutes compiling the dms-core-gate bits a second time.

Testing
linux-pkg build job
Downloaded the zip file built by the job, loaded the images in the zip into k8s, and made sure the masking application started correctly.

[pzakha]

We do not currently support having multiple appliance packages with the same git-url, however given that this package is not in the appliance that should be fine. With that said we should probably add a comment above DEFAULT_PACKAGE_GIT_URL explaining that so someone else doesn't get inspired by this way of doing things.