Enable Dependabot for Docker, GH Actions and Go

This enables Dependabot using three groups, one for container images,
one for GitHub Actions, and one for Go Modules.

In the future, we may want to split the Go Modules into multiple
groups. For example, one for each key source with a misc catch-all
group for any other dependency.

xref: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/customizing-dependency-updates#grouping-dependabot-version-updates-into-one-pull-request

Signed-off-by: Hidde Beydals <hidde@hhh.computer>

.github/dependabot.yaml

@@ -0,0 +1,41 @@
+version: 2
+
+updates:
+  - package-ecosystem: "docker"
+    directory: "/"
+    labels: ["dependencies"]
+    schedule:
+      # By default, this will be on a Monday.
+      interval: "weekly"
+    groups:
+      # Group all updates together, so that they are all applied in a single PR.
+      # xref: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#groups
+      docker:
+          patterns:
+          - "*"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    labels: ["area/CI", "dependencies"]
+    schedule:
+      # By default, this will be on a Monday.
+      interval: "weekly"
+    groups:
+      # Group all updates together, so that they are all applied in a single PR.
+      # xref: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#groups
+      ci:
+        patterns:
+          - "*"
+
+  - package-ecosystem: "gomod"
+    directory: "/"
+    labels: ["dependencies"]
+    schedule:
+      # By default, this will be on a Monday.
+      interval: "weekly"
+    groups:
+      # Group all updates together, so that they are all applied in a single PR.
+      # xref: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#groups
+      go:
+        patterns:
+          - "*"