Rework UI/UX of workers definition

[bohdan-shulha]

Sometimes workers require defining a separate docker image, while they still need to access the exact same volumes as a main process. This is the case for VictoriaMetrics backups.

Re-make workers' definition UI, so that it is possible to define all options (image, healthcheck, command) for individual workers like it is possible to to for the main process.

Consider removing a separate "backup" switch for Volumes. Anyways, app templates should pre-define backup workers and the majority of custom containers wouldn't have any local storage.

[bohdan-shulha]

Slightly better now.