fix(Dockerfile): upgrade golang in upstream Dockerfile

[flavianmissi]

This matches the golang version with Dockerfile @ master. Project doesn't build with with golang 1.13, see kubernetes-sigs/cluster-api#4096

FTR, here's the error:

Step 10/15 : RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 GO111MODULE=on go build -mod vendor -o manager main.go
 ---> Running in 7b4f402b9966
# sigs.k8s.io/controller-runtime/pkg/manager
vendor/sigs.k8s.io/controller-runtime/pkg/manager/manager.go:53:2: duplicate method Start
The command '/bin/sh -c CGO_ENABLED=0 GOOS=linux GOARCH=amd64 GO111MODULE=on go build -mod vendor -o manager main.go' returned a non-zero code: 2

Error: Process completed with exit code 2.

[harishsurf]

Thanks for fixing this, LGTM!