Add internal build support to .NET Monitor Dockerfile templates

[lbussell]

See #6252 for context.

[lbussell]

[Triage] @wiktork, do you have a sense of whether .NET Monitor 10.0 will need internal/MSRC builds for RC1/RC2/GA? That will inform how/when we need to invest in this.

Given monitor is framework-dependent, we only need to do this if any packages it relies on have security fixes. If we don't do this, Monitor can still ship security updates on release days, but we'll just need to build the images after the core .NET images release instead of staging the new images beforehand.

[lbussell]

Fixed for .NET Monitor 10+ with a combination of #6634 and #6789. If for some reason we need this capability for .NET Monitor 8 and 9 then it should be filed as a separate issue.