Bugfix and refactor the docker build.

- Correctly identify the executable to build and copy.
- Order layers more efficiently (don't repeat commands that rarely
  change and take significant time).

Dockerfile

 FROM golang:1.10.3 as builder
-COPY . /go/src/code.uplex.de/uplex-varnish/k8s-ingress
 ARG PACKAGES
 RUN go get -d -v github.com/slimhazard/gogitversion && \
     cd /go/src/github.com/slimhazard/gogitversion && \
-    make install && \
-    cd /go/src/code.uplex.de/uplex-varnish/k8s-ingress && \
-    go get -v $PACKAGES && \
-    go generate && \
-    CGO_ENABLED=0 GOOS=linux go build -o k8-ingress *.go
+    make install
+RUN go get -v $PACKAGES
+COPY . /go/src/code.uplex.de/uplex-varnish/k8s-ingress
+WORKDIR /go/src/code.uplex.de/uplex-varnish/k8s-ingress
+RUN go generate && \
+    CGO_ENABLED=0 GOOS=linux go build -o k8s-ingress *.go
 
 FROM centos:centos7
 COPY varnishcache_varnish60.repo /etc/yum.repos.d/
@@ -18,5 +18,5 @@ RUN yum install -y epel-release && yum update -y -q && \
     yum install -y -q --nogpgcheck vmod-re2-1.5.1 && \
     yum clean all && rm -rf /var/cache/yum
 COPY varnish/vcl/vcl.tmpl /
-COPY --from=builder /go/src/code.uplex.de/uplex-varnish/k8s-ingress .
-ENTRYPOINT ["./k8s-ingress"]
+COPY --from=builder /go/src/code.uplex.de/uplex-varnish/k8s-ingress/k8s-ingress /k8s-ingress
+ENTRYPOINT ["/k8s-ingress"]