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

[chrmarti]

Refs: devcontainers/spec#63

• anyOS @lszomoru
• anyOS @aeschli

Complexity: 2

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Use the latest Remote-Containers pre-release (currently 0.248.0) or Dev Containers CLI (currently 0.12.1) for testing.

A devcontainer.json's property values can reference environment variables locally and inside the container. Test that the updated syntax supports having a default value that is used when the environment variable is not set: ${localEnv:ENV_VAR:default_value} and ${containerEnv:ENV_VAR:default_value}

[lszomoru]

Works as advertised 🎉