Dockerfile.dev: add a faster Dockerfile for developer use-cases

The production Dockerfile can take upwards of 20-30 minutes for
to build. The new Dockerfile.dev allows a developer to quickly build a
derivative image with new dependencies, Mantle and COSA without having
to wait.

Dockerfile.dev

@@ -0,0 +1,27 @@
+# Instead of doing a slow, whole-cloth build for developer
+# tests, you can quickly test changes via this file. This
+# especially useful for changes to dependencies or for final
+# verification of COSA code.
+#
+# This moves the build time from ~15 minutes to ~3-5 minutes.
+# Example: buildah bud -f Dockerfile.dev localhost/cosa-test .
+
+FROM quay.io/coreos-assembler/coreos-assembler:latest
+WORKDIR /root/containerbuild
+
+USER root
+RUN rm -rfv /lib/coreos-assembler /usr/bin/coreos-assembler
+
+COPY ./src/cmdlib.sh ./build.sh ./deps*.txt ./vmdeps.txt ./build-deps.txt /root/containerbuild/
+RUN ./build.sh install_rpms
+
+COPY ./ /root/containerbuild/
+RUN ./build.sh write_archive_info
+RUN ./build.sh make_and_makeinstall
+
+WORKDIR /srv/
+RUN rm -rf /root/containerbuild
+
+# run as `builder` user
+USER builder
+ENTRYPOINT ["/usr/bin/dumb-init", "/usr/bin/coreos-assembler"]