Bump Docker image versions

```improvement developer
The alpine version has been updated to `v3.13.7`.
```
```improvement developer
The Golang version has been updated to `v1.17.6`.
```

Dockerfile

@@ -1,5 +1,5 @@
 ############# builder            #############
-FROM eu.gcr.io/gardener-project/3rd/golang:1.16.7 AS builder
+FROM golang:1.17.6 AS builder
 
 WORKDIR /go/src/github.com/gardener/cloud-provider-gcp
 COPY . .
@@ -9,7 +9,7 @@ RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go install \
   ./...
 
 ############# cloud-provider-gcp #############
-FROM eu.gcr.io/gardener-project/3rd/alpine:3.13.5 AS cloud-provider-gcp
+FROM alpine:3.13.7 AS cloud-provider-gcp
 
 COPY --from=builder /go/bin/gcp-cloud-controller-manager /gcp-cloud-controller-manager