Using docker volumes in packer build

Question

Is it possible to use existing docker or external volumes in/during packer build?

I saw in https://www.packer.io/docs/builders/docker/:

"VOLUME /test1 /test2"

What does it exactly mean? "VOLUME String EX: "VOLUME FROM TO"" doesn't explain much. Is /test1 from host?

I also saw in https://www.packer.io/docs/builders/docker/#volumes:

volumes (map[string]string) - A mapping of additional volumes to mount into this container. The key of the object is the host path, the value is the container path.

How can I make use of that? Where/how can I put/declare it , suppose that I want to map /etc/dnsmasq.d/ host path into the container, during build time and run time as well?

Answer 1

It has the same meaning as the corresponding Dockerfile directive (indeed, all of the directives in that section of the Packer documentation are Dockerfile commands). You probably don't need or want it.

This is different from the docker run -v option to mount content into a container. You cannot specify mount options like this at container build time (whether using docker build or Packer). You don't need to specify a VOLUME to be able to mount content on some container directory.

The Dockerfile VOLUME directive isn't needed for most common uses and mostly only has confusing side effects. You do not need it to mount configuration into your application; you do not need it to overwrite application source code with a development tree; the most obvious thing it does do is prevent future RUN instructions from having an effect. I'd avoid it unless you understand in detail what it does and why you want it.