Add semaphore workflow for npmjs deployment (#48)

.semaphore/deploy_dockerhub.yml

@@ -0,0 +1,21 @@
+version: v1.0
+name: Deploy to DockerHub
+blocks:
+  - name: Deploy to DockerHub
+    task:
+      jobs:
+        - name: Deploy
+          commands:
+            - checkout
+            - 'docker build . -t "devicefarmer/stf:${SEMAPHORE_GIT_TAG_NAME:-latest}"'
+            - docker push devicefarmer/stf
+      secrets:
+        - name: dockerhub-secrets
+      prologue:
+        commands:
+          - 'echo "${DOCKER_PASSWORD}" | docker login -u "${DOCKER_USERNAME}" --password-stdin'
+    dependencies: []
+agent:
+  machine:
+    type: e1-standard-2
+    os_image: ubuntu1804