docker: fix image prune

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

src/docker.js

@@ -26,6 +26,8 @@ exports = module.exports = {
 
     update,
 
+    parseImageName,
+
     createExec,
     startExec,
     getExec,
@@ -42,6 +44,7 @@ const apps = require('./apps.js'),
     promiseRetry = require('./promise-retry.js'),
     services = require('./services.js'),
     settings = require('./settings.js'),
+    semver = require('semver'),
     shell = require('./shell.js'),
     safe = require('safetydance'),
     system = require('./system.js'),
@@ -671,3 +674,11 @@ async function setRegistryConfig(registryConfig) {
 
     await settings.setJson(settings.REGISTRY_CONFIG_KEY, registryConfig);
 }
+
+function parseImageName(imageName) {
+    const repository = imageName.split(':', 1)[0];
+    const tag = imageName.substr(repository.length + 1).split('@', 1)[0];
+    const digest = imageName.substr(repository.length + 1 + tag.length + 1).split(':', 2)[1];
+
+    return { repository, tag, version: semver.parse(tag), digest };
+}