Add dryrun to docker step to test docker image build on PRs

.drone.starlark

@@ -180,6 +180,23 @@ def docker(arch):
           }
         ]
       },
+      {
+        'name': 'dryrun',
+        'image': 'plugins/docker:18.09',
+        'pull': 'always',
+        'settings': {
+          'dry_run': True,
+          'tags': 'linux-%s' % arch,
+          'auto_tag_suffix': 'linux-%s' % arch,
+          'dockerfile': 'docker/Dockerfile.linux.%s' % arch,
+          'repo': 'owncloud/reva-phoenix',
+        },
+        'when': {
+          'event': [
+            'pull_request'
+          ]
+        }
+      },
       {
         'name': 'docker',
         'image': 'plugins/docker:18.09',

.drone.yml

@@ -107,6 +107,19 @@ steps:
   - name: gopath
     path: /srv/app
 
+- name: dryrun
+  pull: always
+  image: plugins/docker:18.09
+  settings:
+    auto_tag_suffix: linux-amd64
+    dockerfile: docker/Dockerfile.linux.amd64
+    dry_run: true
+    repo: owncloud/reva-phoenix
+    tags: linux-amd64
+  when:
+    event:
+    - pull_request
+
 - name: docker
   pull: always
   image: plugins/docker:18.09
@@ -165,6 +178,19 @@ steps:
   - name: gopath
     path: /srv/app
 
+- name: dryrun
+  pull: always
+  image: plugins/docker:18.09
+  settings:
+    auto_tag_suffix: linux-arm64
+    dockerfile: docker/Dockerfile.linux.arm64
+    dry_run: true
+    repo: owncloud/reva-phoenix
+    tags: linux-arm64
+  when:
+    event:
+    - pull_request
+
 - name: docker
   pull: always
   image: plugins/docker:18.09
@@ -223,6 +249,19 @@ steps:
   - name: gopath
     path: /srv/app
 
+- name: dryrun
+  pull: always
+  image: plugins/docker:18.09
+  settings:
+    auto_tag_suffix: linux-arm
+    dockerfile: docker/Dockerfile.linux.arm
+    dry_run: true
+    repo: owncloud/reva-phoenix
+    tags: linux-arm
+  when:
+    event:
+    - pull_request
+
 - name: docker
   pull: always
   image: plugins/docker:18.09