Fix docker file, add test suite

Dockerfile

@@ -1,13 +1,26 @@
 ARG VERSION=3
 FROM python:$VERSION
 
-RUN mkdir /isort
-WORKDIR /isort
+# Install pip and poetry
+RUN python -m pip install --upgrade pip && python -m pip install poetry
 
+# Setup as minimal a stub project as posible, simply to allow caching base dependencies
+# between builds.
+#
+# If error is encountered in these steps, can safely be removed locally.
+RUN mkdir -p /isort/isort
+RUN mkdir -p /isort/tests
+RUN touch /isort/isort/__init__.py
+RUN touch /isort/tests/__init__.py
+RUN touch /isort/README.md
+WORKDIR /isort
 COPY pyproject.toml poetry.lock /isort/
-RUN python -m pip install --upgrade pip && python -m pip install poetry && poetry install
+RUN poetry install
 
-COPY . /isort/
+# Install latest code for actual project
+RUN rm -rf /isort
+COPY . /isort
 RUN poetry install
 
+# Run full test suite
 CMD /isort/scripts/test.sh