Variable substitution does not allow default values containing a colon

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`