support custom docker images

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Rally-core has additional needs. Rather than update the image for all addons, let's allow for a docker image name in the manifest, which points at a docker-image name.

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I'm thinking we can add a node-15 docker-image task, and make sure we can point various builds at either 14 or 15.
This probably means for release, we'll need to add a manifest option to override the default, and for CI, we'll need to look at package.json for a potential default override.

Related: https://github.com/mozilla-extensions/xpi-manifest/blob/master/taskcluster/ci/docker-image/kind.yml#L13-L21
https://github.com/mozilla-extensions/xpi-manifest/blob/master/taskcluster/ci/build/kind.yml#L18