further enhancements

[line-o]

This is a followup on #15

A few enhancements we should consider now or later:

drop .existdb.json

Put all necessary package metadata in a property app, exist or xar in package.json.
npm packages are implicitly allowed to add their custom properties in package.json but have to take care themselves not to clash with names used by npm itself. I have use app for other projects in the past.

add npm script to install the library (without the test application)

I usually use npm start for that.

optimise GithubActions

• Use prebuilt docker images (gives cacheing of images for free, makes the test preparation and workflow definition less complex and error prone).
• As npm test already calls gulp install:all copying the XAR into auto deploy of the docker container is superfluous
• Also test on docker image tag 5.3.0 to ensure backwards compatibility for future versions of this lib

adopt readOptionsFromEnv

To allow all npm and gulp scripts to target different existdb instances with ease.

A quite complete setup with all of the above can be found in a eeditiones/roaster#30 which is not yet merged.