Parameterize Unikraft build

[ls-1801]

Just FYI, I build the docker image locally and pushed it to the google cloud registry:

europe-docker.pkg.dev/bdspro/eu.gcr.io/unikraft-gcp-image-builder:latest

here is the script I use to launch the builder container in gcp. You should implement that in python, I guess: I also update the build_unikraft.sh to stop the instance on exit.

ZONE="us-west3-b"
PROJECT_ID="bdspro"

gcloud compute instances create-with-container unikraft-builder \
                --zone="$ZONE" \
                --project="$PROJECT_ID" \
                --container-env=ZONE="$ZONE" \
                --container-env=PROJECT_ID="$PROJECT_ID" \
                --container-image "europe-docker.pkg.dev/bdspro/eu.gcr.io/unikraft-gcp-image-builder:latest" \
                --container-arg "new-image" \
                --container-arg "<GITHUB_TOKEN_FOR_PRIVAT_REPO>" \
                --scopes=https://www.googleapis.com/auth/compute,https://www.googleapis.com/auth/servicecontrol,https://www.googleapis.com/auth/service.management.readonly,https://www.googleapis.com/auth/logging.write,https://www.googleapis.com/auth/monitoring.write,https://www.googleapis.com/auth/trace.append,https://www.googleapis.com/auth/devstorage.read_write

[fschmllr]

Can you also specify the changes that would need to apply when following the dynamic image resolution according to #66