Add docker multiarch manifest build step

.drone.yml

@@ -282,3 +282,31 @@ steps:
       auto_tag: true
       auto_tag_suffix: linux-amd64
     depends_on: [ fetch-tags ]
+
+---
+kind: pipeline
+type: docker
+name: docker-manifest
+
+trigger:
+  ref:
+    - refs/heads/master
+    - "refs/tags/**"
+
+depends_on:
+  - docker-amd64-latest
+  - docker-arm-latest
+
+steps:
+  - name: manifest
+    pull: always
+    image: plugins/manifest
+    settings:
+      auto_tag: true
+      ignore_missing: true
+      spec: docker-manifest.tmpl
+      password:
+        from_secret: docker_password
+      username:
+        from_secret: docker_username
+

docker-manifest.tmpl

@@ -0,0 +1,18 @@
+image: vikunja/frontend:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}latest{{/if}}
+{{#if build.tags}}
+tags:
+{{#each build.tags}}
+  - {{this}}
+{{/each}}
+{{/if}}
+manifests:
+  -
+    image: vikunja/frontend:{{#if build.tag}}{{trimPrefix "v" build.tag}}-{{/if}}linux-amd64
+    platform:
+      architecture: amd64
+      os: linux
+  -
+    image: vikunja/frontend:{{#if build.tag}}{{trimPrefix "v" build.tag}}-{{/if}}linux-arm
+    platform:
+      architecture: arm
+      os: linux