CICD: Add Dependency Scanning / Updates by Dependabot.

Add scanning and updates of dependencies with Dependabot where this is
possible.

While this isn't currently available for the majority of our code that
is in C / C++, there are some things 'around the edges' where we can
offload some of the dependency management:
- GitHub actions
- docker images
- the gradle build for android

Signed-off-by: Michael Keller <github@ike.ch>

.github/dependabot.yml

@@ -0,0 +1,19 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    # Workflow files stored in the default location of `.github/workflows`. (You don't need to specify `/.github/workflows` for `directory`. You can use `directory: "/"`.)
+    directory: "/"
+    schedule:
+      interval: "weekly"
+
+  - package-ecosystem: "gradle"
+    directory: "/android-mobile"
+    schedule:
+      interval: "weekly"
+
+  - package-ecosystem: "docker"
+    directories:
+      - "/scripts/docker/android-build-container"
+      - "/scripts/docker/mxe-build-container"
+    schedule:
+      interval: "weekly"