Add an extra docker-compose intermediate step

[nimpo]

When adding build to the docked compose. I suggest perhaps showing how one might use the new image built in the previous steps via the image keyword thus:
What was:

services:
  spuc:
    image: spuacv/spuc:latest
...
    volumes:
    ...
      - ./stats.py:/spuc/plugins/stats.py

which fails as the stats.py requires pandas.
Do this as the next step first, using what we just learned:

services:
  spuc:
    image: spuc-stats

then move onto.

services:
  spuc:
    build:
      context: .
      dockerfile: Dockerfile

Then replace that with the build: tag.

---

NB

  build:
    context: .
    dockerfile: Dockerfile

is equivalent to

  build: .