im using this to build my image

// Step 3: Build and publish a Docker image to a private ECR registry.
const ApiImage = awsx.ecs.Image.fromDockerBuild('api-image', {
  context: path.resolve(__dirname, '../../../../'),
  dockerfile: path.resolve(__dirname, '../../', 'Dockerfile'),
});

Is there a way to make it build a new docker image only if the content of the context has changed ? like storing a SHA256 hash of the folder contents and checking against it to decide to build or reuse last one ?

I don't think there is, within the Pulumi logic. I think I found a GitHub issue about this, a while ago.