Build mounts guide + `rw` "discard" behavior

[milas]

Is this a docs issue?

• My issue is about the documentation content or website

Type of issue

I can't find what I'm looking for

Description

In the Dockerfile reference @ https://docs.docker.com/reference/dockerfile/#run---mounttypebind, the docs for the rw param mention:

Written data will be discarded.

This is not covered anywhere in the guide for build mounts, which leads to the assumption that they behave like run mounts, which are bi-directional.

Location

https://docs.docker.com/build/guide/mounts/

Suggestion

Add a short explanation about the use-case for read-write mounts with clarification that they do NOT propagate back to the host and that all "written data will be discarded".

[dvdksn]

Cheers @milas - good callout! I agree that this deserves clarification, it's in my todo pile.

[docker-robot]

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