docker: Use secret for ROSIPW on RHEL7 build dockerfile

[sxa]

Current process exposes the ROSI credential in the docker image. This switches the process to use a secret supplied in an environment variable as per https://docs.docker.com/build/building/secrets

Another option would be to use a secret file (e.g. secret id=ROSPIW,src=$HOME/.rosipw) but that runs a great risk of being accessed if cleanup does not occur properly.

Draft while testing is performed, but reviews welcome in the meantime

Checklist

• commit message has one of the standard prefixes
• faq.md updated if appropriate
• other documentation is changed or added (if applicable)
• playbook changes run through VPC or QPC (if you have access)
• VPC/QPC not applicable for this PR
• for inventory.yml changes, bastillion/nagios/jenkins updated accordingly

[sxa]

Ran into various weird certificate problems using the old version of the docker image I had on the machine (sha256:0d98d4a2076fbf871a4854f05f2f83ee673cc619e6a741edde9886771561fc13). The new one (sha256:7df1d6982eed0c3ce48699b9a9fb0dcb3706db5417a87bda6f9a916a9e0a8bf8) does not exhibit the same problem, so I believe this is good to go. Removing from draft.

[steelhead31]

LGTM