Add drone CI starlark file

.drone.star

@@ -0,0 +1,36 @@
+# Automatically builds docker images and pushes them to the registry.
+# Required drone secrets:
+#   - docker_repo       eg. 'xeniaproject/buildenv'
+#   - docker_username   user with write permissions to repo
+#   - docker_password   password of the user
+
+def main(ctx):
+    return [
+        pipeline('2021-05-05'),
+        pipeline('2021-06-21'),
+        pipeline('2021-09-29'),
+    ]
+
+def pipeline(tag):
+    return {
+        'kind': 'pipeline',
+        'type': 'docker',
+        'name': tag,
+        'steps': [
+            {
+                'name': 'docker',
+                'image': 'plugins/docker',
+                'settings': {
+                  'dockerfile': 'Dockerfile.' + tag,
+                  'tags': [tag],
+                  'repo': {'from_secret': 'docker_repo'},
+                  'username': {'from_secret': 'docker_username'},
+                  'password': {'from_secret': 'docker_password'},
+                },
+            },
+        ],
+        'trigger': {
+            'branch': ['master'],
+            'event': ['push']
+        },
+    }