Build multi-arch image

[fbuetler]

Currently, strimzi-canary 0.6.0 is not built as multi-arch:

➜ docker manifest inspect quay.io/strimzi/canary:0.6.0
WARN[0001] The manifest type application/vnd.docker.distribution.manifest.v2+json is not a manifest list but a single image.
{
    "schemaVersion": 2,
    "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
    "config": {
        "mediaType": "application/vnd.docker.container.image.v1+json",
        "size": 1532,
        "digest": "sha256:cfffb5785a5e6961baa366e5ddaffb73a024f7a00f4a76c9a1ff20c418816018"
    },
    "layers": [
        {
            "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
            "size": 11235815,
            "digest": "sha256:a0eb529af5c3b6661b7fa399bae8722a8179b761085712b1ea2fa224a5902401"
        }
    ]
}