fix: remove apt cache layer (#136)

Dockerfile

@@ -1,13 +1,12 @@
-from golang:1.11 as build
+FROM golang:1.11 as build
 
 ENV SRC github.com/segmentio/aws-okta
 ARG VERSION
 
 WORKDIR /build
 COPY . /go/src/${SRC}
 
-RUN apt-get update
-RUN apt-get install --no-install-recommends -y \
+RUN apt-get update && apt-get install --no-install-recommends -y \
     libusb-1.0-0-dev \
     ca-certificates \
     build-essential \
@@ -18,4 +17,3 @@ RUN CGO_ENABLED=1 go build -o aws-okta -ldflags="-X main.version=$VERSION" ${SRC
 FROM scratch
 COPY --from=build /build/aws-okta /
 ENTRYPOINT ["/aws-okta"]
-