Use ephemeral EC2 runners for building multi-arch docker images

[rbuysse]

[rbuysse]

This branch was pushed to upstream rather than my fork to demonstrate that the secrets are in place to allow the job to run. See https://github.com/hyperledger/grid/runs/6838377090?check_suite_focus=true for a successful build of grid-dev.

[CLEckhardt]

Do not merge until we want to use these runners.