build: Dockerfile for the builder image

Dockerfile.builder

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+# It's difficult to install snapcraft (the packaging tool) on a non-snap
+# distribution such as Debian without running snapd (which doesn't work under
+# Docker). We cheat by taking their Docker image and snarfing the resulting
+# binaries.
+FROM snapcore/snapcraft AS snapcraft
+
+# Otherwise we base on the Go image, which is Debian based.
+FROM golang
+
+# The snap stuff, as mentioned, with variables set to let the snapcraft snap play ball
+COPY --from=snapcraft /snap /snap
+ENV LANG="en_US.UTF-8"
+ENV LANGUAGE="en_US:en"
+ENV LC_ALL="en_US.UTF-8"
+ENV SNAP="/snap/snapcraft/current"
+ENV SNAP_NAME="snapcraft"
+ENV SNAP_ARCH="amd64"
+ENV PATH="$PATH:/snap/bin"
+
+# FPM to build Debian packages
+RUN apt-get update && apt-get install -y --no-install-recommends \
+	locales rubygems ruby-dev \
+	&& apt-get clean \
+	&& rm -rf /var/lib/apt/lists/* \
+	&& gem install --no-ri --no-rdoc fpm \
+	&& locale-gen en_US.UTF-8