rust-python docker build docs

.circleci/docker/README.md

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+# Rust-Python CircleCI Container
+
+
+### Setup
+```bash
+# the default username in the container
+# circle ci expects the user "circleci"
+export DOCKER_USER=<USERNAME>
+
+# the version of python to build
+export PYTHON_VERSION=<VERSION>
+```
+
+### Build
+```bash
+# pass local env vars as build args USER and VERSION
+docker build -t nucypher/rust-python:$PYTHON_VERSION . \
+--build-arg VERSION=${PYTHON_VERSION} \
+--build-arg USER=${DOCKER_USER}
+```
+
+### Push
+```bash
+# docker login or authentication required
+docker push nucypher/rust-python:$PYTHON_VERSION
+```