Add black Dockerfile (#1916)

* Add `black` Dockerfile
- Build a `black` container based on python3-slim
- Test: `docker build --network-host --tag black .`
```console
cooper-mbp1:black cooper$ docker image ls
REPOSITORY                                         TAG            IMAGE ID       CREATED              SIZE
black                                              latest         0c7636402ac2   4 seconds ago        332MB
```

Addresses part of #1914

* Build with colorama + d extra installs - Adds ~9mb

* Combine all build commands in 1 run
- Combine the commands all into 1 RUN to down the size
- Get to 65.98 MB image size now: https://hub.docker.com/repository/docker/cooperlees/black/tags?page=1&ordering=last_updated

* Add rm -r of /var/lib/apt/lists/* + save 10mb down to 55mb

Dockerfile

@@ -0,0 +1,14 @@
+FROM python:3-slim
+
+RUN mkdir /src
+COPY . /src/
+RUN pip install --no-cache-dir --upgrade pip setuptools wheel \
+    && apt update && apt install -y git \
+    && cd /src \
+    && pip install --no-cache-dir .[colorama,d] \
+    && rm -rf /src \
+    && apt remove -y git \
+    && apt autoremove -y \
+    && rm -rf /var/lib/apt/lists/*
+
+CMD ["black"]