Build multi-arch images locally, including ARM and RISC-V

[chazapis]

This pull request builds upon #40, allowing the use of make for building multi-arch images locally and pushing them to the repository. ARM, ARM64 and RISC-V support is included, but other architectures can be easily added in the BUILD_ARCH variable. The base container has been changed to Alpine (to support RISC-V), which has also made the resulting images even more compact.