build: cache go mod (#345)

For local development, to remove the step where go modules are
downloaded on every `docker build`, first copy only `go.mod` and
`go.sum` to the Docker workspace.

This step only needs to be performed whenever one of those files
changes.

Signed-off-by: Blake Pettersson <blake.pettersson@gmail.com>

Dockerfile

@@ -4,15 +4,14 @@ ARG LD_FLAGS='-s -w -linkmode external -extldflags "-static"'
 ARG TARGETPLATFORM
 
 WORKDIR /app
-COPY . .
 
 RUN export GOOS=$(echo ${TARGETPLATFORM} | cut -d / -f1) && \
     export GOARCH=$(echo ${TARGETPLATFORM} | cut -d / -f2)
 
-RUN go env
+COPY go.* ./
+RUN go env && go mod download
 
-RUN go get -d -v \
-    && go install -v
+COPY . .
 
 RUN CGO_ENABLED=1 go build -ldflags="${LD_FLAGS}" -tags="sqlite_unlock_notify" -o /app/build/policyreporter -v