Optimize Dockerfile

Dockerfile had a step for caching dependencies (go mod download). However, this is already done by the vendor directory, which we include. Therefore, this step can be removed to make the image build time faster.

Dockerfile

@@ -7,9 +7,6 @@ WORKDIR /workspace
 # Copy the Go Modules manifests
 COPY go.mod go.mod
 COPY go.sum go.sum
-# cache deps before building and copying source so that we don't need to re-download as much
-# and so that source changes don't invalidate our downloaded layer
-RUN go mod download
 
 # Copy the go source
 COPY cmd/main.go cmd/main.go