Update to golang@1.15.9 (gardener#3688)

Signed-off-by: ialidzhikov <i.alidjikov@gmail.com>

Dockerfile

@@ -1,6 +1,6 @@
 #############      builder       #############
 ARG GCR_PULL_URL=eu.gcr.io/gardener-project/3rd/
-FROM ${GCR_PULL_URL}golang:1.15.7 AS builder
+FROM ${GCR_PULL_URL}golang:1.15.9 AS builder
 
 WORKDIR /go/src/github.com/gardener/gardener
 COPY . .
@@ -10,7 +10,7 @@ ARG EFFECTIVE_VERSION
 RUN make install EFFECTIVE_VERSION=$EFFECTIVE_VERSION
 
 ############# base
-FROM ${GCR_PULL_URL}alpine:3.12.3 AS base
+FROM ${GCR_PULL_URL}alpine:3.13.2 AS base
 
 #############      apiserver     #############
 FROM base AS apiserver