container build optimization

if the required docker image already exists remotely then pull that one instead of building it locally i.e. don't build locally over what has already been published (semver must always increase with changes)
- requires a more permanent container registry for CD output
- probably blocked by #7 or a similar story that sets up the container registry for CD (rather than the container registry in terraform from #54 )
move the COPY src after the RUN npm install layer so that if there are changes to the code files but not to the package.json then the relatively slow install step won't be run again (but if there are new packages installed or other package.json changes then it will be rerun)
make the local build scripts aware of pending changes; if the local package.json version is not in the remote (prod) repository then rebuild the image even if it already exists locally

hypnopotamus / SkillsMatrix