Enable dependabot updates (#13976)

Fixes https://github.com/matrix-org/synapse/issues/11828

.github/dependabot.yml

@@ -0,0 +1,17 @@
+version: 2
+updates:
+  - # "pip" is the correct setting for poetry, per https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem
+    package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+
+  - package-ecosystem: "docker"
+    directory: "/docker"
+    schedule:
+      interval: "weekly"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"

changelog.d/13976.misc

@@ -0,0 +1 @@
+Enable update notifications from Github's dependabot.