add multi-arch docker support

[jawabuu]

Following implementation of pomerium/pomerium#1643
multi-arch images for pomerium operator should also be published.
Otherwise this breaks in systems with arm architecture when operator is enabled in the helm install.
Related issue
pomerium/pomerium#268
@travisgroth @desimone

[travisgroth]

Closed by #143

This should be resolved by migrating to the Pomerium Ingress Controller (docs). See deprecation notice for more info.