Support ARM Docker images

[dominikbraun]

Since Apple M1, people are talking about ARM - and so should we :-)

verless needs support for ARM-based Docker images distributed within the CD process. There are some ways to accomplish this, including:

• Download and install buildx as a step in the CD pipeline and cross-build the ARM image
• Use a CircleCI Orb for that task
• Or: Spin up an ARM-based VM in the pipeline and build the image natively. Whether this makes sense depends on the support and stability of buildx.