232T & docker script refactoring

[CarlosNZ]

Includes changes required for https://github.com/msupply-foundation/conforma-templates/issues/232, plus some improvements to the docker scripts and the updateRowPolicies.

Summary of changes

• New Action: "generateTextFile" which allows us to output JSON/text files to anywhere on the system
• New Action: "getValues" -- the simplest action yet, but also probably one of the most useful. If we're querying the same value in several different actions in sequence, then it makes sense to just fetch this value once and pass it along to subsequent actions in the output. This makes the evaluator expressions for subsequent actions way simpler. In this case, I wanted to query the next available port number from the database and it seemed silly (not to mention tedious and error-prone) to do it 3 or 4 different times in sequence.
• Update Dockerfile to be able to specify a snapshot -- if defined the image will be built with this as the base snapshot rather than "core_templates"
• Get rid of the unnecessary "updateRowPolicies" function -- all it did was query an endpoint which in turned called the actual "updateRowPolicies" function. Have added updateRowPolicies() to server start (it was already in snapshot load), so we no longer need the separate command line version of if either as part of the docker scripts
• Made environment variables available to "applicationData" -- but only those explicitly defined in server preferences envVars field

[andreievg]

I also didn't quite see how initial snapshot is loaded within docker

[CarlosNZ]

Approving but pending the getValue doc and some reference to the config.envVars (and potentially renaming of that to specify what it means)

Cool, have addressed those. Thanks for taking a look. Will merge now.