Docker run --mount make all files available in a different folder during RUN

I want to make a folder on my host machine available during a RUN statement. That is, similar to the effect of a container run with -v:

docker run -v /path/on/host:/path/in/container mycontainer:tag

In the container this gives me /path/in/container with all files/folder in path/on/host.

To this end I am trying the experimental mount options from https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/experimental.md:

RUN --mount=type=bind,target=/path/on/host

This gives me a folder /path/on/host during the RUN.

I then have two problems:

I can ls files inside /path/on/host, but not use them (e.g. cat them). I have tried changing type to e.g. cache and using source like in https://devops.stackexchange.com/questions/6078/in-a-dockerfile-is-there-a-way-to-avoid-copying-files-to-make-them-accessible-t, but I cannot make it work.
I cannot figure out how to have a different path inside the "RUN image", that is, /path/in/container instead of /path/on/host.

Solution

I think you have misunderstood what the RUN --mount=type=bind... syntax is for. From the documentation:

This mount type allows binding directories (read-only) in the context or in an image to the build container.

In other words, this does not permit you to access arbitrary host directories in the build stage. It is not an analog to the -v command line option on docker run. It only permits you to:

Mount directories from your build context, or
Mount directories from another stage in a multi-stage build

So for example I can do this do mount a directory from one build stage into a subsequent build stage:

# syntax=docker/dockerfile:experimental

FROM centos AS centos

FROM alpine
RUN --mount=type=bind,from=centos,source=/,target=/centos ls /centos > /root/centos.txt

Or if I have a directory named example in my build context, I can do this to mount it during the build process:

# syntax=docker/dockerfile:experimental

FROM centos AS centos

FROM alpine
RUN --mount=type=bind,source=example,target=/data cp /data/* /root/

The syntax you're using (with no from specified)...

RUN --mount=type=bind,target=/path/on/host

...simply mounts the root of your build context on /path/on/host inside the container. Remember that target specifies the mountpoint inside the container. E.g., if my build context looks like this:

.
├── Dockerfile
└── example
    └── README.md

And example/README.md contains:

This is a test.

And the Dockerfile contains a RUN option similar to what you're using:

# syntax=docker/dockerfile:experimental

FROM centos AS centos

FROM alpine
RUN --mount=type=bind,target=/data cat /data/example/README.md > /root/README.md

Then when the image is built, /root/README.md has the contents of example/README.md.