develop is the branch where people land when they visit GitHub - frappe/frappe_docker: Docker images for production and development setups of the Frappe framework and ERPNext. It is also default branch when repo is cloned.
branches have significance for travis builds here. Not for code.
The important code, Dockerfile(s), config, scripts etc are exactly same on both branches. It is kept same.
master branch triggers versioned image builds. develop branch triggers develop/edge image builds on travis