No arm64 Docker images

[bbenligiray]

We don't build arm64 Docker images, which means the users can't run the containers on ARM machines. An example scenario is the user wanting to run airnode-client on an ARM EC2 instance.

After some cursory research, I'm not convinced that there will be a significant performance gain from running Airnode on arm64 instead of x86_64 (at the same price point). That's not a reason to not log the issue though.