with-cache

#!/bin/bash
#
# Extracts the cache date from the registry images and provides them into the
# environment so local images may be built with caching even on the first time.
#
set -euo pipefail

main() {
    local registry tag

    # Set default values.
    registry=docker.io
    tag=latest

    # Read command-line arguments.
    while getopts "r:t:" opt; do
        case "$opt" in
            r) registry="$OPTARG";;
            t) tag="$OPTARG";;
            *) echo "Usage: $0 [-r <registry>] [-t <tag>]" 1>&2; exit 1;;
        esac
    done

    image="docker://$registry/nextstrain/base-builder:$tag"

    log "Looking up CACHE_DATE for $image"
    date="$(cache-date "$image")"

    log "Exporting CACHE_DATE=$date"
    export CACHE_DATE="$date"

    log "Running your command now…"
    exec "$@"
}

cache-date() {
    local image="$1"
    skopeo inspect --config "$image" | jq --raw-output '
          .history 
        | map(.created_by)
        | map(match("\\bCACHE_DATE=(\\S+)\\b") | .captures[0].string)
        | .[0]
    '
}

skopeo() {
    docker run \
        --rm \
        --network host \
        --volume "$HOME"/.docker/config.json:/auth.json:ro \
            quay.io/skopeo/stable "$@"
}

log() {
    echo "==> $*" >&2
}

main "$@"