Fix docker build after removal of vendored deps

I believe the docker build broke after merging #93. This change should
fix #105

Dockerfile

@@ -6,12 +6,20 @@
 # To run certstrap from the image (for example):
 #     docker run --rm squareup/certstrap --version
 
-FROM golang:1.11.2-alpine as build
+FROM golang:1.13-alpine as build
 
 MAINTAINER Cedric Staub "[email protected]"
 
+WORKDIR /app
+
+COPY go.mod .
+COPY go.sum .
+
+# Download dependencies
+RUN go mod download
+
 # Copy source
-COPY . /go/src/github.com/square/certstrap
+COPY . .
 
 # Build
 RUN go build -o /usr/bin/certstrap github.com/square/certstrap
@@ -21,4 +29,4 @@ FROM alpine
 
 COPY --from=build /usr/bin/certstrap /usr/bin/certstrap
 
-ENTRYPOINT ["/usr/bin/certstrap"]
+ENTRYPOINT ["/usr/bin/certstrap"]