Setting DEBIAN_FRONTEND in docker image is not a good practice

[code-R]

moby/moby#4032

Use arg instead of setting as env DEBIAN_FRONTEND
https://github.com/att-comdev/drydock/blob/master/images/drydock/Dockerfile#L16

This issue will be common for most of the UCP docker images.

[sh8121att]

Thanks for this pointer, we'll look at making this change.