Add digest for base image and configure dependabot

This should not change the current behavior, but will cause dependabot
to update the digest of the latest base image weekly.

Signed-off-by: Wes Panther <wpanther@google.com>

.github/dependabot.yml

@@ -19,3 +19,8 @@ updates:
       interval: weekly
     allow:
       - dependency-type: all
+
+  - package-ecosystem: docker
+    directory: /
+    schedule:
+      interval: weekly

Dockerfile

@@ -1,5 +1,5 @@
 ARG ARCH=amd64
-FROM --platform=linux/${ARCH} gcr.io/distroless/static-debian12
+FROM --platform=linux/${ARCH} gcr.io/distroless/static-debian12@sha256:4a2c1a51ae5e10ec4758a0f981be3ce5d6ac55445828463fce8dff3a355e0b75
 
 ADD etcd /usr/local/bin/
 ADD etcdctl /usr/local/bin/