This fixes currently failing Docker builds. kemalcr/kemal in version 0.26.0 requires Crystal 0.30.0 which is not yet available on Alpine 3.10 (previously used as the Docker base image).
This includes the following changes: - Use multi-stage build to run application in an optimized environment, see https://docs.docker.com/develop/develop-images/multistage-build/ - Run application on alpine instead of archlinux to further reduce image size - Build Crystal application with --release for improved runtime performance - Run application as non-root user for better security, see https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#user - Only rebuild Docker layers when required