Dockerfile should first copy requirements.txt and install dependencies before copying agent code

[idofl]

adk-python/src/google/adk/cli/cli_deploy.py

Line 203 in 0dbc37d

# Copy agent - Start

In the current structure of the Dockerfile, the build first creates a layer with the agent's code, and then creates another layer with the installed dependencies. This order of operations causes the build process to re-create the dependencies layer after every code change, even if the list of dependencies in the requirements.txt hasn't changed.

The correct order should be:

1. copy only requirements.txt file.
2. run pip install to download dependencies
3. copy agent code

This should reduce the amount of time it takes to build container images when redeploying updated agent code, as long as the dependencies list doesn't change.

[mikemikimike]

I'm preparing a focused fix for this issue: generate the Dockerfile by copying an agent's requirements.txt and installing those dependencies before copying the rest of the agent source, so source-only changes can reuse the dependency layer. I'll include a regression test that verifies the generated instruction order and will keep the change limited to the shared deployment template.