Is your feature request related to a problem? Please describe.
As a contributor to the ADK framework, I'm unable to test my local, unreleased code changes with the adk deploy gke command. The command currently only supports installing the google-adk version from PyPI, which makes it impossible to validate new features in a real GKE environment before they are merged and released.

Describe the solution you'd like
I'd like the adk deploy gke cmd to include a flag, such as --editable. When this flag is present, the deployment process should build the container using the local ADK source code from the current working directory, rather than installing the package from PyPI. The default behavior without the flag should remain unchanged.

Describe alternatives you've considered
The only alternative is to manually edit the cli_deploy.py script to change the Dockerfile generation logic for every local test. This is an error-prone and unsustainable workflow that must be reverted before committing any changes.

Additional context
Adding this feature will significantly improve the development and testing loop for ADK contributors. It enables proper end-to-end validation of new features in a production-like environment, leading to more robust contributions.