Improve CI

Add more tests to the CI, including a docker build test, and add the
generation and publication of a new docker image on master and tags

.drone.yml

@@ -3,12 +3,23 @@ type: docker
 name: Tests
 
 steps:
-- name: test
+- name: Go tests
   image: golang
   commands:
-  - cd tracker
-  - make build
-  - make test
+  - make -C tracker test
+
+- name: Go build
+  image: golang
+  commands:
+  - make -C tracker build
+
+- name: Docker build
+  image: thegeeklab/drone-docker-buildx
+  privileged: true
+  settings:
+    platforms:
+      - linux/amd64
+      - linux/arm64
 
 trigger:
   event: