chore: add Hadolint Dockerfile linting

* as per #22
* update PR pipeline in .drone.yml
* add .hadolint.yaml

.drone.yml

@@ -24,6 +24,12 @@ steps:
   commands:
     - uname -r
 
+- name: hadolint
+  pull: always
+  image: hadolint/hadolint:latest
+  commands:
+  - uname -r
+  - hadolint --version
 
 ---
 kind: pipeline
@@ -68,6 +74,12 @@ depends_on:
 - build
 
 steps:
+- name: hadolint
+  image: hadolint/hadolint:latest
+  commands:
+  - hadolint --version
+  - hadolint Dockerfile
+
 - name: build
   pull: always
   image: tmaier/docker-compose:latest

.hadolint.yaml

@@ -0,0 +1,2 @@
+ignored:
+  - DL3007