Add ability for optional builder config placeholders

[mih]

With this change a builder recipe template can employ placeholders for which defaults can be made available, hence enabling optional customization without forcing additional complexity on users.

With the new setup, both of the following call would work with the same default singulairty recipe template

datalad deb-configure-builder dockerbase=mike

datalad deb-configure-builder dockerbase=mike "debian_archive_sections=main contrib non-free"

This is an alternative to #97 which required a copy of the recipe template to achieve the same.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.29%. Comparing base (3f26c4c) to head (fe881af). Report is 51 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #108 +/- ## ========================================== + Coverage 93.27% 93.29% +0.01% ========================================== Files 16 16 Lines 491 492 +1 ========================================== + Hits 458 459 +1 Misses 33 33 ```

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