Rework the worker due to `Dataset` API changes

Closes #14 (closed), #16 (closed)

ci/build.sh

 #!/bin/sh -e
 # Build the tasks Docker image.
 # Requires CI_PROJECT_DIR and CI_REGISTRY_IMAGE to be set.
-# VERSION defaults to latest.
 # Will automatically login to a registry if CI_REGISTRY, CI_REGISTRY_USER and CI_REGISTRY_PASSWORD are set.
 # Will only push an image if $CI_REGISTRY is set.
 
-if [ -z "$VERSION" -o -z "$CI_PROJECT_DIR" -o -z "$CI_REGISTRY_IMAGE" ]; then
+if [ -z "$VERSION" ] || [ -z "$CI_PROJECT_DIR" ] || [ -z "$CI_REGISTRY_IMAGE" ]; then
   echo Missing environment variables
   exit 1
 fi
 
 IMAGE_TAG="$CI_REGISTRY_IMAGE:$VERSION"
 
-cd $CI_PROJECT_DIR
+cd "$CI_PROJECT_DIR"
 docker build -f Dockerfile . -t "$IMAGE_TAG"
 
-if [ -n "$CI_REGISTRY" -a -n "$CI_REGISTRY_USER" -a -n "$CI_REGISTRY_PASSWORD" ]; then
-  echo $CI_REGISTRY_PASSWORD | docker login -u $CI_REGISTRY_USER --password-stdin $CI_REGISTRY
-  docker push $IMAGE_TAG
+if [ -n "$CI_REGISTRY" ] && [ -n "$CI_REGISTRY_USER" ] && [ -n "$CI_REGISTRY_PASSWORD" ]; then
+  echo "$CI_REGISTRY_PASSWORD" | docker login -u "$CI_REGISTRY_USER" --password-stdin "$CI_REGISTRY"
+  docker push "$IMAGE_TAG"
 else
   echo "Missing environment variables to log in to the container registry…"
 fi