Add platform arg to run container options

[Copilot]

This PR adds support for specifying a target platform when running containers, similar to how it's already supported for building images.

The changes include:

• Added a platform?: ContainerPlatform; property to RunContainerCommandOptions interface
• Updated getRunContainerCommandArgs method to use withDockerPlatformArg(options.platform)
• Added test to verify the platform flag is properly included in the command

This allows users to specify the target platform directly through the API instead of having to construct it through CLI strings manually or use customOptions (which doesn't parse correctly for this use case).

Example usage:

const client = new DockerClient();
await client.runContainer({
  imageRef: 'myimage',
  platform: {
    os: 'linux',
    architecture: 'arm64'
  }
});
// Generates: docker container run --platform linux/arm64 myimage

Fixes #258.

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