Add a "production build" version for docker images

[rolfkleef]

Minimise the production image size:

• Exclude tests/ and lib/xspec
• Exclude docs/

Check docker for options to do this (env parameters to select what to copy into the image, or use a different .dockerignore?)

[rolfkleef]

Also useful to then add the template sheets to this repository (and exclude them perhaps from both builds).