can't set environment variables from stdout of a command

[airtonix]

version := ${$(git rev-parse --short HEAD):develop}

docker.build:
  
  # pull the cached layers 
  docker pull ${repo}:builder || true

  # rebuild it. if nothing in these layers
  # have changes, this should be very fast.
  docker build \
  --cache-from ${repo}:builder \
  --file ./tools/docker/app/Dockerfile \
  --tag ${repo}:builder \
  --target install \
  .

  # # pull the latest image
  docker pull ${repo}:latest || true

  # rebuild it using cache layers from:
  # - builder
  # - latest
  docker build \
  --build-arg COMMIT=${version} \
  --cache-from ${repo}:builder \
  --cache-from ${repo}:latest \
  --file ./tools/docker/app/Dockerfile \
  --tag ${repo} \
  --target prod \
  .