docker: fix image prune

it seems docker images --digests cloudron/sftp --format "{{.ID}} {{.Repository}}:{{.Tag}}@{{.Digest}}
broke at some point

src/test/check-install

@@ -47,7 +47,7 @@ if [[ ${#missing_scripts[@]} -gt 0 ]]; then
     exit 1
 fi
 
-images=$(node -e "let i = require('${SOURCE_DIR}/src/infra_version.js'); console.log(Object.keys(i.images).map(function (x) { return i.images[x].tag; }).join('\n'));"; echo $TEST_IMAGE)
+images=$(node -e "const i = require('${box_src_tmp_dir}/src/infra_version.js'); console.log(Object.keys(i.images).map(x => i.images[x]).join(' '));")
 
 for image in ${images}; do
     if ! docker inspect "${image}" >/dev/null 2>/dev/null; then