Add dependabot config

Add a dependabot config adapted from the one in element-hq/synapse.

Author: anoadragon453

.github/dependabot.yml

@@ -0,0 +1,75 @@
+# This file is adapted from the same file in element-hq/synapse:
+# https://github.com/element-hq/synapse/blob/2e9b8202f0a1a8ceba9f02bb5ec227498d51dcbd/.github/dependabot.yml
+# But updated to remove `pip` and `cargo` ecosystems, and introduce `gomod`.
+version: 2
+# As dependabot is currently only run on a weekly basis, we raise the
+# open-pull-requests-limit to 10 (from the default of 5) to better ensure we
+# don't continuously grow a backlog of updates.
+updates:
+    package-ecosystem: "gomod"
+    directory: "/"
+    open-pull-requests-limit: 10
+    schedule:
+      interval: "weekly"
+    # Group patch updates to packages together into a single PR, as they rarely
+    # if ever contain breaking changes that need to be reviewed separately.
+    # 
+    # Less PRs means a streamlined review process.
+    #
+    # The Go ecosystem is special in that breaking changes are often introduced
+    # in minor version bumps, as packages typically stay pre-1.0 for a long time.
+    # Thus we specifically keep minor version bumps separate in their own PRs.
+    groups:
+      minor-and-patches:
+        applies-to: version-updates
+        patterns:
+          - "*"
+        update-types:
+          - "minor"
+          - "patch"
+    # Prevent pulling packages that were recently updated to help mitigate
+    # supply chain attacks. 14 days was taken from the recommendation at
+    # https://blog.yossarian.net/2025/11/21/We-should-all-be-using-dependency-cooldowns
+    # where the author noted that 9/10 attacks would have been mitigated by a
+    # two week cooldown.
+    #
+    # The cooldown only applies to general updates; security updates will still
+    # be pulled in as soon as possible.
+    cooldown:
+      default-days: 14
+
+  - package-ecosystem: "docker"
+    directory: "/docker"
+    open-pull-requests-limit: 10
+    schedule:
+      interval: "weekly"
+    # For container versions, breaking changes are also typically only introduced in major
+    # package bumps.
+    groups:
+      minor-and-patches:
+        applies-to: version-updates
+        patterns:
+          - "*"
+        update-types:
+          - "minor"
+          - "patch"
+    cooldown:
+      default-days: 14
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    open-pull-requests-limit: 10
+    schedule:
+      interval: "weekly"
+    # Similarly for GitHub Actions, breaking changes are typically only introduced in major
+    # package bumps.
+    groups:
+      minor-and-patches:
+        applies-to: version-updates
+        patterns:
+          - "*"
+        update-types:
+          - "minor"
+          - "patch"
+    cooldown:
+      default-days: 14