Allow for default value in ${localEnv:...} and ${containerEnv:...}

[chrmarti]

Goal

String values in the devcontainer.json can contain ${localEnv:ENV_VAR} and ${containerEnv:ENV_VAR} placeholders which are replaced with the (either the local or the container) ENV_VAR environment variable's value at runtime. The goal is to allow for a default value to be specified as part of the syntax that will be used when the ENV_VAR is not set.

Proposal

The proposal is to add an optional default value separated by a second colon in the existing notation: ${localEnv:ENV_VAR:default_value} and ${containerEnv:ENV_VAR:default_value} and to replace the notation with the default value when there is no ENV_VAR environment variable at runtime. The existing behavior without a default value remains unchanged.

Notes

We have a PR from the community implementing this proposal: devcontainers/cli#51

[chrmarti]

On the potential need to escape : when it is part of an environment variable name: I can't find a way to use : as part of an environment variable's name in Bash or /bin/sh. When spawning a process without shell, it is possible to use : in a variable name. Given the difficulty in using it (i.e., unsupported in common shells), I suggest we don't need to support it. /cc @jkeech

(REMOVED: The original ask was to use :- as the separator to align with common shell syntax. Updating to that.)

[chrmarti]

Spec PR: #64