Add option to run `npm run migrate_db` via a Docker ENV variable

[jason-fox]

Proposed changes

Add option to run npm run migrate_db via a Docker ENV variable

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• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

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[jason-fox]

Related to #216 - this will help with migration as it implements the simple quick fix suggestion made by @aarranz in https://github.com/ging/fiware-idm/issues/216#issuecomment-861312772

Allowing Keyrock to automigrate is potentially dangerous and not foolproof as it relies on the list of previously applied fixes to be up to date. It should be highly recommended that a backup is taken first.

• IDM_DB_MIGRATE defaults to false and must be supplied from the config
• If IDM_DB_MIGRATE is true, seeding will not run
• If IDM_DB_SEED is true it will create, migrate and seed the database as before
• If neither IDM_DB_SEED nor IDM_DB_MIGRATE is true no changes will be attempted

Note: neither IDM_DB_MIGRATE nor IDM_DB_SEED is supported in distroless builds since they currently rely on exec() to run sequelize which in turn requires a command line.

[apozohue10]

Thanks!