Pull image during `enclaver build` if it's not on local disk

[robszumski]

On a machine with no docker images, using this config file:

version: v1
image: "us-docker.pkg.dev/edgebit-containers/containers/no-fly-list:4ea61b5"
name: "example-enclave"

I would expect for my image to be pull since it doesn't exist locally:

$ sudo ./enclaver build -f enclaver.yaml --eif-only app.eif
error: Docker responded with status code 404: no such image: us-docker.pkg.dev/edgebit-containers/containers/no-fly-list:4ea61b5: No such image: us-docker.pkg.dev/edgebit-containers/containers/no-fly-list:4ea61b5

[robszumski]

Fixed in #57

$ enclaver build --pull