[Enhancement] Add option for using pre-built docker image (or pass custom arguments to `docker build` command)

We need dependencies from our internally hosted package repository on our runtimes which requires credentials to fetch. We pass in these credentials using the `--secret` flag when we build images ourselves.

Currently, there is no way (that I can see) to run `agentcore launch --local(-build)` with a pre-built image or pass custom arguments to the `docker build` command. Either would be required for us to be able to use this tool to launch runtimes. Line 313 in `operations/runtime/launch.py` has the comment "# Step 1: Build Docker image (only if we need it)", but I don't see that any checks for existing images actually happen.

Start with operations/runtime/launch.py around line 313 and trace how `agentcore launch --local` and `--local-build` construct the Docker build command. Determine how an existing image or custom `docker build` arguments would be accepted, including the reported `--secret` use case; done means local launches can use either supported option without manually building outside the toolkit.

Written by the indexing model from the issue text.