Replace Travis with Github workflows

[benediktschwab]

A growing number of laptop models are switching from x86 to ARM-based processor architectures due to improved power efficiency. Operating systems can usually emulate x86, but with performance penalties.

Hence, it would be beneficial, if the Docker image of the 3D City Database Web-Map-Client also supports the ARM-based architecture (linux/arm64). The Node.js base image is already offered for multiple architectures.