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`

Start by tracing the localEnv and containerEnv substitution handling and reproduce the provided WEBSITE_URL example. Check how colon-separated default values are parsed. Done means an unset variable preserves the complete default value, including https://example.com, with regression coverage for the reported case.

Scritto dal modello di indicizzazione a partire dal testo della issue.