Define builder dataset layout

[mih]

The purpose of this dataset is to provide one or more containerized environment for building packages for a specific distribution and target CPU architecture.

This issue is picking up the definition used by #12 (comment) and adds detail and further definitions.

Key structure of a builder dataset

./
   recipes/
      <target-name>.def/
   envs/
      <target-name>_<architecture>

• /recipes contains the (actionable) description for (re)creating a build environment (e.g., a singularity recipe). Conceptually, this is not limited to container recipes, but this is the first implementation goal.
• /envs generated build environments. Importantly, the analog build environment for different CPU architectures must be able to co-exist.
• Compatible environments in envs are registered with the tooling of the datalad-containers extension.