To further her add to what Peter is saying - in GitLab you can add secret variables (which are encrypted) and can be referenced only during the build process.
So you can call your image with the value of one of these variables. Which is fine if you don't persist the value.
I'm wondering if we have a concept of "transient" variables that don't persist on image save? Then iceberg could use one of those, and expect you to pass that value through on the command line of Pharo? I was toying with the idea of having an encrypted vault in my image (to hold multiple values for convenience) and I would then pass an unlock value on the command line - but I would need some way to ensure that value isn't persisted? Is this where a plugin might help? Something to save a value transiently only during image execution?