Add docker release pipeline

.drone.docker.yml

@@ -0,0 +1,21 @@
+kind: pipeline
+name: default
+
+steps:
+- name: build
+  image: r.pkg.cx/build/go:1.14
+  pull: true
+  commands:
+    - "go build -v -ldflags \"-X main.version=${DRONE_COMMIT_SHA:0:8}\" -a -tags netgo -o release/linux/amd64/drone-gitea-release"
+
+- name: docker
+  image: plugins/docker
+  settings:
+    username:
+      from_secret: docker_username
+    password:
+      from_secret: docker_password
+    dockerfile: docker/Dockerfile.linux.amd64
+    registry: r.pkg.cx
+    repo: r.pkg.cx/drone/gitea-release
+    auto_tag: true