Move stuff to .gitlabci.yml and simplify Dockerfile

Dockerfile

@@ -1,14 +1,3 @@
-FROM golang:1.12 as deps
-WORKDIR /build
-COPY go.mod go.sum ./
-RUN go get -mod=readonly
-
-FROM golang:1.12 as builder
-WORKDIR /build
-COPY * ./
-RUN CGO_ENABLED=0 GOOS=linux go build -mod=readonly -a -installsuffix cgo -o default-request-adder .
-
 FROM scratch
-COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
-COPY --from=builder /build/default-request-adder /
+COPY default-request-adder /
 CMD ["/default-request-adder"]