[Feature] allow for build-args and other arguments for local image build

We have our dependencies in an internal repository which requires authentication.
Currently is there is no way to expose them to the image build without explicitly including them in the docker file.

Proposing build args to the cli launch command: `/bedrock_agentcore_starter_toolkit/cli/runtime/commands.py`, something like:
```python
# add to launch arguments

build_arguments: Optional[List[str]] = typer.Option(
None, "--build-args", "-ba", help="Build arguments for agent (format: KEY=VALUE)"
)

# add to launch function

build_args = None
if build_arguments:
build_args = {}
for build_arg in build_arguments:
if "=" not in build_arg:
_handle_error(f"Invalid build argument variable format: {build_arg}. Use KEY=VALUE format.")
key, value = build_arg.split("=", 1)
build_args[key] = value

# Call the operation - CodeBuild is now default, unless --local-build is specified
result = launch_bedrock_agentcore(
config_path=config_path,
agent_name=agent,
local=local,
use_codebuild=not local_build,
env_vars=env_vars,
auto_update_on_conflict=auto_update_on_conflict,
build_args=build_args,
)
```

and update `/bedrock_agentcore_starter_toolkit/utils/runtime/container.py
```python
def build(self, dockerfile_dir: Path, tag: str, platform: Optional[str] = None, build_args: Optional[dict[str, str]] = None) -> Tuple[bool, List[str]]:

...

cmd = [self.runtime, "build", "-t", tag]
build_platform = platform or self.DEFAULT_PLATFORM
cmd.extend(["--platform", build_platform])
cmd.extend([x for k, v in build_args.items() for x in ("--build-arg", k, v)])
cmd.append(str(dockerfile_dir))

```

Start in bedrock_agentcore_starter_toolkit/cli/runtime/commands.py and trace the launch command through to launch_bedrock_agentcore, then read bedrock_agentcore_starter_toolkit/utils/runtime/container.py and its build method. Done means the CLI accepts KEY=VALUE build arguments, rejects invalid formats, and forwards valid arguments to the local image build command.

Written by the indexing model from the issue text.