As we are preparing to open up features to community contributions, we found that some features need to install after other features. This issue proposes to add support for soft-dependencies to arrive at a reproducible ordering following these ordering requirements:

A feature can declare a list of other features that should be installed first if they are configured to be installed. (The soft-dependency itself does not add another feature to the list of features to be installed.)

E.g., the NodeJS feature would list the Python feature because it has to install a system version of Python if no version of Python is already installed:

{
    "id": "node",
    "installAfter": [ "devcontainers/features/python" ],
    // ...
}

Features that an install in any order will be installed in a fixed order (e.g., ordered by id) to ensure the result is always the same.

The "features" map in the devcontainer.json will remain a map. See https://github.com/devcontainers/spec/pull/27/files#r869018039 for a discussion.

To allow for progress on user contributable features, we can use a stop-gap measure like an ordered list of features that depend on a specific installation order and either bake that into the CLI at build time or fetch it at runtime. This list will be temporary and documented as such.

/cc @Chuxel @jkeech @edgonmsft @joshspicer @bamurtaugh @alexdima