fix build for go 1.15

[mwennrich]

• call go mod vendor from Makefile
• use code-generator from go.mod
• update to golang:1.15 AS builder
• update to alpine:3.12
• update dependencies

[Gerrit91]

This looks promising. But the Makefile target generate-in-docker, that we require every now and then, does not work for me:

$ make generate-in-docker 
...
Generate
cp: cannot create regular file '../../../hack/../vendor/k8s.io/code-generator/': No such file or directory
bash: ../../../hack/../vendor/k8s.io/code-generator/generate-internal-groups.sh: No such file or directory
bash: ../../../hack/../vendor/k8s.io/code-generator/generate-internal-groups.sh: No such file or directory
bash: ../../../hack/../vendor/k8s.io/code-generator/generate-internal-groups.sh: No such file or directory
bash: ../../../hack/../vendor/k8s.io/code-generator/generate-internal-groups.sh: No such file or directory
pkg/apis/metal/doc.go:18: running "../../../hack/generate-code": exit status 127
Successfully generated controller registration at ../../example/controller-registration.yaml
...