Variable substitution does not allow default values containing a colon

[grthr]

The current implementation does not support default values in localEnv or containerEnv substitution that include a colon.

Example:

 "WEBSITE_URL": "${localEnv:WEBSITE_URL:https://example.com"

Current behavior: If $WEBSITE_URL is not present, the variable is just set to https

Expected behavior: If $WEBSITE_URL is not present, the variable should be set to https://example.com

[grthr]

PR #882