chore(deps): update docker.io/ruby docker tag to v4

Author: renovate[bot]

docker/Dockerfile.ruby

@@ -19,7 +19,7 @@ RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} \
   go build -a -ldflags "-s -w -X github.com/target/flottbot/version.Version=${VERSION}" \
   -o flottbot ./cmd/flottbot
 
-FROM docker.io/ruby:3.4.7-alpine@sha256:cdb113e8110f2ed8c5e2e1ef4b02494e0243f8d6fe59e4c0348441437ded2aaa
+FROM docker.io/ruby:4.0.0-alpine@sha256:375f2a0e8ced42cab1b6b89a60d4509544845199a63131e73d8ce45562b05ad4
 
 ENV USERNAME=flottbot
 ENV GROUP=flottbot