Don't rely on COMPOSE_PROJECT_IMAGE to set the pipeline code image in the docker example
Summary: The current way of specifying the pipeline image is complicated and error-prone. Set the image name manually instead of relying on the compose project name.
Test Plan: BK, run example locally
Reviewers: alangenfeld, prha, sashank, johann
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D7294