[BUG] Don't overwrite existing Dockerfile

**Describe the bug**
I want to use the CLI for config management and deployment, but I want to use my own Dockerfile and I'm not using the SDK wrapper for FastAPI

**To Reproduce**
Normal setup to config an agent

**Expected behavior**
Check if I already have a Dockerfile, if yes, don't overwrite. Instead, maybe warn the user that the Dockerfile exists and can't guarantee it will work as well as the jinja2 dockerfile template.

Locate the CLI path used by the normal agent setup that generates the Dockerfile, then reproduce the reported configuration flow with an existing Dockerfile. Done means the existing file is preserved and the user receives a warning that the custom Dockerfile may not work like the Jinja2 template.

Written by the indexing model from the issue text.