CICD: prebuild docker image for test hardware

A large amount of time is spent in installing the toolchains directly on the Raspberry Pi that does the hardware testing. Now we build a Docker container for ARM which has already prebundled all those tools. To do so, we use buildx together with a dummy pio run command; that is necessary because pio platform install does not install optional packages and does not offer any option to select or specify the correct version for those.