Switch Dockerfile.e2e to alpine 3.8

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Dockerfile.e2e

@@ -1,5 +1,5 @@
 ## Step 1: Build tests
-FROM golang:1.11.4-alpine3.7 as builder
+FROM golang:1.11.4-alpine3.8 as builder
 
 RUN apk --no-cache add \
     bash \
@@ -39,7 +39,7 @@ RUN hack/make.sh build-integration-test-binary
 RUN mkdir -p /output/tests && find . -name test.main -exec cp --parents '{}' /output/tests \;
 
 ## Step 2: Generate testing image
-FROM alpine:3.7 as runner
+FROM alpine:3.8 as runner
 
 # GNU tar is used for generating the emptyfs image
 RUN apk --no-cache add \