`.dockerignore`: Use an allowlist approach for selective builds

For very selective builds, a robust strategy is to ignore everything by default (*) and then use the negation pattern (!) to explicitly include only the necessary files or directories.

Example:

```
# Ignore everything
*
# Include only what's needed
!src/
!package.json
!package-lock.json
```

Seems it has not been updated for 8 years:

https://github.com/apache/cloudstack/blob/main/.dockerignore

Start with the repository's root .dockerignore, which the issue identifies as outdated, and compare its current patterns with the proposed allowlist example. Verify that the resulting exclusions still permit the selective builds and required paths described in the issue.

Written by the indexing model from the issue text.