Allow steps to be injected before ImageBuilder runs

[premun]

Backport of dotnet/dotnet-buildtools-prereqs-docker#1729 where we needed to copy eng/common in the right time but there was no hook for it yet

[lbussell]

@premun the addition of this hook is probably fine, but would #1592 have eliminated the need for this? It would allow you to set build context to the root of the repo, for example.

[premun]

@lbussell probably yes, the original problem was we could not copy the root-level eng/common.