Add a docker recipe

Based on the closest Alpine Erlang version currently supported by
rebar3. Allows people to mix and match different versions of
the Erlang VM and rebar3 using multi-stage builds.

.dockerignore

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+.DS_Store
+.gitignore
+_build
+_checkouts
+Dockerfile
+.dockerignore
+

Dockerfile

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+# https://docs.docker.com/engine/reference/builder/#from
+#   "The FROM instruction initializes a new build stage and sets the
+#    Base Image for subsequent instructions."
+FROM erlang:20.3.8.1-alpine as builder
+# https://docs.docker.com/engine/reference/builder/#label
+#   "The LABEL instruction adds metadata to an image."
+LABEL stage=builder
+
+# Install git for fetching non-hex depenencies. Also allows rebar3
+# to find it's own git version.
+# Add any other Alpine libraries needed to compile the project here.
+# See https://wiki.alpinelinux.org/wiki/Local_APK_cache for details
+# on the local cache and need for the symlink
+RUN ln -s /var/cache/apk /etc/apk/cache && \
+    apk update && \
+    apk add --update openssh-client git 
+
+# WORKDIR is located in the image
+#   https://docs.docker.com/engine/reference/builder/#workdir
+WORKDIR /root/rebar3
+
+# copy the entire src over and build
+COPY . .
+RUN ./bootstrap
+
+# this is the final runner layer, notice how it diverges from the original erlang
+# alpine layer, this means this layer won't have any of the other stuff that was
+# generated previously (deps, build, etc)
+FROM erlang:20.3.8.1-alpine as runner
+
+# copy the generated `rebar3` binary over here
+COPY --from=builder /root/rebar3/_build/prod/bin/rebar3 .
+
+# and install it
+RUN HOME=/opt ./rebar3 local install \
+    && ln /opt/.cache/rebar3/bin/rebar3 /usr/local/bin/rebar3 \
+    && rm -rf rebar3
+
+# simply print out the version for visibility
+ENTRYPOINT ["/usr/local/bin/rebar3"]
+CMD ["--version"]
+