Fix bin/build-image.sh on Linux (#1131)

Without the `DOCKER_BUILDKIT` environment variable the image fails to
build on Linux (note: this doesn't affect CI because CI doesn't use this
helper script).

bin/build-image.sh

@@ -7,4 +7,6 @@ QUESMA_BUILD_SHA=$(git rev-parse HEAD)
 QUESMA_BUILD_DATE=$(git --no-pager log -1 --date=format:'%Y-%m-%d' --format="%ad")
 QUESMA_VERSION="development"
 
+export DOCKER_BUILDKIT=1
+
 docker build --build-arg QUESMA_BUILD_DATE="$QUESMA_BUILD_DATE" --build-arg QUESMA_VERSION="$QUESMA_VERSION" --build-arg QUESMA_BUILD_SHA="$QUESMA_BUILD_SHA" -f quesma/Dockerfile -t quesma/quesma:nightly quesma