ci: group Dependabot security updates and Docker image bumps (#1546)

Author: lhoupert

.github/dependabot.yml

@@ -27,6 +27,12 @@ updates:
       all:
         patterns:
         - "*"
+      # `all` only applies to version updates, so without this each security
+      # advisory opens its own PR.
+      security:
+        applies-to: security-updates
+        patterns:
+        - "*"
     commit-message:
       prefix: "chore(deps):"
 
@@ -36,3 +42,7 @@ updates:
     directory: "/dockerfiles"
     schedule:
       interval: "weekly"
+    groups:
+      all:
+        patterns:
+        - "*"