You can use a premade Docker image to build yourself an OpenWrt firmware image. A list of known such Docker images (see instructions how to use them on their pages):
To facilitate easy sharing, reuse, and swapping of builders and testing out of new firmwares in the wider OpenWrt community, the following standard is proposed.
To facilitate sharing an project specific OpenWRT buildroot in such a way non Docker or Linux experts can understand. https://github.com/Doodle3D/openwrt-buildroot-example