Update Dockerfile to get dependencies before copying files

This should speed up build time on updates.

Dockerfile

@@ -1,11 +1,13 @@
 FROM golang:1.14-alpine
 
 WORKDIR /go/src/Goff
-COPY . .
+COPY ./go.mod .
 
 RUN apk add --no-cache git
 
 RUN go get -d -v ./...
+
+COPY . .
 RUN go install -v ./...
 
 ENTRYPOINT /go/bin/goff