better docker file caching; added a note about pushed image

Dockerfile

@@ -18,7 +18,6 @@ RUN go build -o packer-builder-arm-image
 
 FROM ubuntu:eoan
 
-COPY --from=builder /build/packer-builder-arm-image /bin/packer-builder-arm-image
 RUN apt-get update -qq \
  && DEBIAN_FRONTEND=noninteractive apt-get install -qqy \
   qemu-user-static \
@@ -37,4 +36,6 @@ RUN wget https://github.com/hashicorp/packer/releases/download/nightly/packer_li
   rm /tmp/packer.zip
 WORKDIR /build
 COPY entrypoint.sh /entrypoint.sh
+
+COPY --from=builder /build/packer-builder-arm-image /bin/packer-builder-arm-image
 ENTRYPOINT ["/entrypoint.sh"]

README.md

@@ -107,12 +107,23 @@ docker build -t packer-builder-arm .
 docker run \
   --rm \
   --privileged \
-  -v ${PWD}:/build \
+  -v ${PWD}:/build:ro \
   -v ${PWD}/packer_cache:/build/packer_cache \
-  -v ${PWD}/output:/build/output \
+  -v ${PWD}/output-arm-image:/build/output-arm-image \
   packer-builder-arm build samples/raspbian_golang.json
 ```
 
+Alternativly, you can use the `docker.pkg.github.com/solo-io/packer-builder-arm-image/packer-builder-arm` that's built off latest master:
+```
+docker run \
+  --rm \
+  --privileged \
+  -v ${PWD}:/build:ro \
+  -v ${PWD}/packer_cache:/build/packer_cache \
+  -v ${PWD}/output-arm-image:/build/output-arm-image \
+  docker.pkg.github.com/solo-io/packer-builder-arm-image/packer-builder-arm build samples/raspbian_golang.json
+```
+
 - Flashing your image
 
 Follow the examples in the **Flashing** section of this document.