Improve developer workflow and documentation

[denkv]

Currently it mostly says "start gatsby develop" which is suboptimal in many ways:

• Where and how to get gatsby, which version?
• When and whether to run npm install or alternatives?
• Which version of node is required? (it actually failed to install dependencies on both older and newer versions)
• The required version of node may be unavailable on my system.

Maybe there should be a container-based approach for running the development server and building. I am using the following Dockerfile (it does not run the development server, just performs the build):

FROM node:14
RUN npm install -g gatsby-cli
RUN gatsby telemetry --disable
ADD ./scripts/package* /app/scripts/
WORKDIR /app/scripts
RUN npm install
ADD . /app/
RUN gatsby build
WORKDIR /app/scripts/public
EXPOSE 8000
CMD ["python3", "-m", "http.server", "8000"]