feat: use Google managed base images (#2159)

This commit updates the Bullseye and Buster containers to pull from a
Google-managed Debian image.

Context:

Google maintains base images for building its own applications. These
images are built from the same source that Docker Hub uses. Therefore,
they match the images you would get from Docker Hub.

The advantage of using Google-maintained images is that they are stored
on Google Cloud, so you can pull these images directly from your
environment without having to traverse networks.

Google updates these images whenever a new version of an official image
is released and the new version fixes a known vulnerability reported by
Artifact Analysis. For more information on image versions, see the
GitHub repository of official images.

See https://cloud.google.com/software-supply-chain-security/docs/base-images

Dockerfile.bullseye

@@ -26,7 +26,7 @@ RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} \
     go build -ldflags "-X github.com/GoogleCloudPlatform/cloud-sql-proxy/v2/cmd.metadataString=container.bullseye"
 
 # Final stage
-FROM debian:bullseye@sha256:5a87974e73c64b3fb161d444a84bdd47c0e6b6058eacaeea64342e7cbce1f04d
+FROM gcr.io/cloud-marketplace/google/debian11@sha256:97c05bb689107b51ac5c1656d8001db175ee2c214897ab6d51d74dc2a2db5023
 
 LABEL org.opencontainers.image.source="https://github.com/GoogleCloudPlatform/cloud-sql-proxy"
 

Dockerfile.buster

@@ -26,7 +26,7 @@ RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} \
     go build -ldflags "-X github.com/GoogleCloudPlatform/cloud-sql-proxy/v2/cmd.metadataString=container.buster"
 
 # Final stage
-FROM debian:buster@sha256:f6b3b7c7b049c2c7d0f19ae988b4eac64fd8e127fa891c9de1d3cf3f8c33cad4
+FROM gcr.io/cloud-marketplace/google/debian10@sha256:c583011c5370f62f33116fcb214de8c4665b5dc4bf40e2a9157361474647f27f
 
 LABEL org.opencontainers.image.source="https://github.com/GoogleCloudPlatform/cloud-sql-proxy"