Automated multi-platform build and DockerHub publication (#8)

* Automated multi-arch build

This setup uses GitHub Actions to build an image for arm/v7 and arm64 then publish them on DockerHub (you need to setup repo secrets first) every time you want (workflow_dispatch), every push, every pull requests (pull_requests), or on a schedule (cronjob) :)

Remove the triggers you don't want.

Reminder: if you ever move the Dockerfile (or some dependencies), you will have to correct the Dockerfile path (and/or the context path).

* Create dependabot.yml

This checks the dependencies' versions for the Actions everyday and creates a pull request if there are new versions available.

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"