Improve Dockerfile

This commit makes several changes to the dockerfile:

- Add go.mod and go.sum in a seperate stage, subsequently calling `go
  mod download` to make it cache dependencies and speed up builds
- Use ubuntu:latest (28MB larger) instead of scratch, makes the image a
  lot easier to debug (e.g. it has a shell and a package manager)
- Change ENTRYPOINT to CMD, this makes the behaviour of the image
  slightly different from a CLI perspective, but makes interacting with
  the image from code, docker-compose and kubernetes easier.

Dockerfile

@@ -1,12 +1,19 @@
 FROM golang:latest AS build
 ENV GOPATH /go
-COPY . /go/src/headscale
+
+COPY go.mod go.sum /go/src/headscale/
 WORKDIR /go/src/headscale
+RUN go mod download
+
+COPY . /go/src/headscale
+
 RUN go install -a -ldflags="-extldflags=-static" -tags netgo,sqlite_omit_load_extension ./cmd/headscale
 RUN test -e /go/bin/headscale
 
-FROM scratch
-COPY --from=build /go/bin/headscale /go/bin/headscale
+FROM ubuntu:latest
+
+COPY --from=build /go/bin/headscale /usr/local/bin/headscale
 ENV TZ UTC
+
 EXPOSE 8080/tcp
-ENTRYPOINT ["/go/bin/headscale"]
+CMD ["headscale"]