Add support for Dependabot

This commit adds support for Dependabot, which will automatically make
PRs on a weekly basis (if necessary) for GitHub Actions and the
Dockerfile.

.github/dependabot.yml

@@ -0,0 +1,15 @@
+version: 2
+updates:
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "chore(deps)"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "chore(ci)"

.gitignore

@@ -18,6 +18,7 @@
 !.gitignore
 !.gitattributes
 !.mailmap
+!.github
 
 # Backup files
 *~