Bump Docker image versions

```improvement developer
The alpine version has been updated to `v3.12.1`.
```
```improvement developer
The Golang version has been updated to `v1.15.5`.
```

Dockerfile

@@ -1,5 +1,5 @@
 ############# builder            #############
-FROM golang:1.15.0 AS builder
+FROM eu.gcr.io/gardener-project/3rd/golang:1.15.5 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 \
   ./...
 
 ############# base               #############
-FROM alpine:3.12.0 AS base
+FROM eu.gcr.io/gardener-project/3rd/alpine:3.12.1 AS base
 
 RUN apk add --update bash curl