Add Dependabot configuration for GitHub Actions and Docker

Inline base image references in Dockerfile (remove ARG indirection) so Dependabot can detect and propose updates to pinned digests.

Author: dash14

.github/dependabot.yml

@@ -0,0 +1,19 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 7
+
+  - package-ecosystem: "docker"
+    directories:
+      - "/docker"
+      - "/test"
+      - "/test/test-dns"
+      - "/test/test-server"
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 7

docker/Dockerfile

@@ -1,11 +1,7 @@
-ARG BUILDKIT_VERSION=v0.28.0
-ARG BUILDKIT_DIGEST=sha256:37539dd4d60fc70968d164d3850d903a2c56f6402214a1953fbf9fcb81ada731
 ARG CNI_VERSION=v1.9.0
-ARG ALPINE_RELEASE=3.23
-ARG ALPINE_DIGEST=sha256:25109184c71bdad752c8312a8623239686a9a2071e8825f20acb8f2198c3f659
 
 # Prepare dependencies
-FROM alpine:${ALPINE_RELEASE}@${ALPINE_DIGEST} AS deps
+FROM alpine:3.23@sha256:25109184c71bdad752c8312a8623239686a9a2071e8825f20acb8f2198c3f659 AS deps
 ARG CNI_VERSION
 RUN apk add --no-cache curl gettext && \
     mkdir -p /opt/cni/bin && \
@@ -14,7 +10,7 @@ RUN apk add --no-cache curl gettext && \
       | tar -C /opt/cni/bin -xz ./bridge ./host-local ./loopback
 
 # Final image
-FROM moby/buildkit:${BUILDKIT_VERSION}@${BUILDKIT_DIGEST}
+FROM moby/buildkit:v0.28.0@sha256:37539dd4d60fc70968d164d3850d903a2c56f6402214a1953fbf9fcb81ada731
 
 LABEL org.opencontainers.image.title="buildcage" \
       org.opencontainers.image.description="Secure Docker build environment with network access control" \