Upgraded some libraries and required golang version

Dockerfile

-FROM golang:1.14.3-stretch as builder
+FROM golang:1.15.2-buster as builder
 WORKDIR /
 RUN apt-get update && apt-get install unzip
 RUN wget https://github.com/protocolbuffers/protobuf/releases/download/v3.13.0/protoc-3.13.0-linux-x86_64.zip
@@ -16,6 +16,6 @@ WORKDIR /build/pkg/cmd/server
 RUN CGO_ENABLED=0 GOOS=linux go build
 
 FROM alpine:latest
-MAINTAINER Michał Bień
+MAINTAINER Lukasz Lopatowski (llopat@man.poznan.pl)
 COPY --from=builder /build/pkg/cmd/server/server /go/bin/nmaas-janitor
 ENTRYPOINT /go/bin/nmaas-janitor -port $SERVER_PORT -token $GITLAB_TOKEN -url $GITLAB_URL

go.mod

@@ -9,16 +9,16 @@ require (
         github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
         github.com/modern-go/reflect2 v1.0.1 // indirect
         github.com/peterbourgon/diskv v2.0.1+incompatible // indirect
-        github.com/xanzy/go-gitlab v0.37.0
+        github.com/xanzy/go-gitlab v0.38.1
         golang.org/x/net v0.0.0-20200707034311-ab3426394381
         google.golang.org/grpc v1.31.0
         google.golang.org/protobuf v1.24.0
         gopkg.in/inf.v0 v0.9.1 // indirect
         gopkg.in/yaml.v2 v2.3.0
-        k8s.io/api v0.19.0
-        k8s.io/apimachinery v0.19.0
-        k8s.io/client-go v0.19.0
+        k8s.io/api v0.19.2
+        k8s.io/apimachinery v0.19.2
+        k8s.io/client-go v0.19.2
         k8s.io/klog v0.1.0 // indirect
 )
 
-go 1.13
+go 1.15