Configure multi-stage container build incl template

[mattcen]

This in part addresses #4, and removes the (mostly duplicated) dev Dockerfile. Needs testing before being merged.

[mattcen]

To see the benefit of this, see this output:

mattcen@hobart:~$ docker image ls --format='{{.Size}} {{.Repository}}' | grep 'GB ' | sort -h
1.15GB scouthack-website_scouthack_website
1.16GB stemscouts-vic_stemscouts_vic
1.16GB vk3saw-web_vk3saw
1.16GB wvjota-web_wvjota
1.18GB sts_website_sts_website
1.19GB ghcr.io/scouts-victoria-program/scoutsvic-rego
1.2GB ghcr.io/scouts-victoria-program/radioactiv8
1.2GB mattcen/radioactiv8
1.2GB nsw-reg_stem_website

If these were all based off a single image, most of that 1GB would be shared.