[NOD-495] Update dockerfile and initialize go.mod

docker/Dockerfile

@@ -1,28 +1,38 @@
 # -- multistage docker build: stage #1: build stage
 FROM golang:1.13-alpine AS build
+ARG KASPAD_VERSION=latest
 
-RUN mkdir -p /go/src/github.com/kaspanet/kaspad
+RUN mkdir -p /go/src/github.com/kaspanet/dnsseeder
 
-WORKDIR /go/src/github.com/kaspanet/kaspad
+WORKDIR /go/src/github.com/kaspanet/dnsseeder
 
 RUN apk add --no-cache curl git
 
 COPY go.mod .
 COPY go.sum .
 
+# For development it's useful to have kaspad from filesystem, but for deployment
+# we should use the one in github
+RUN go mod edit -dropreplace github.com/kaspanet/kaspad
+RUN go mod edit -require github.com/kaspanet/kaspad@$KASPAD_VERSION
+RUN cp go.mod go.mod.bu
+
 RUN go mod download
 
 COPY . .
 
-RUN cd dnsseeder && CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o dnsseeder .
+# Restore go.mod because `COPY . .` overwrote it
+RUN mv go.mod.bu go.mod
+
+RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o dnsseeder .
 
 # --- multistage docker build: stage #2: runtime image
 FROM alpine
 WORKDIR /app
 
 RUN apk add --no-cache tini
 
-COPY --from=build /go/src/github.com/kaspanet/kaspad/dnsseeder/ /app/
+COPY --from=build /go/src/github.com/kaspanet/dnsseeder/ /app/
 
 ENTRYPOINT ["/sbin/tini", "--"]
 CMD ["/app/dnsseeder"]

go.mod

@@ -0,0 +1,12 @@
+module github.com/kaspanet/dnsseeder
+
+go 1.13
+
+require (
+	github.com/jessevdk/go-flags v1.4.0
+	github.com/kaspanet/kaspad v0.0.0-00010101000000-000000000000
+	github.com/miekg/dns v1.1.25
+	github.com/pkg/errors v0.8.1
+)
+
+replace github.com/kaspanet/kaspad => ../kaspad