Running docker on Ubuntu: mounted host volume is not writable from container

Question

Docker works great on a Mac for me, but I have to run docker host inside of a VirtualBox (or Parallels, or VMWare Fusion), since Mac's kernel doesn't support docker.

So I tried to setup my application and a docker-compose on an Ubuntu Desktop - natively, where both docker client and docker host run physically on the same system. This worked, but my running docker containers can't write into a mounted host volume.

I use docker-compose with the following settings:

volumes:    - ./api:/usr/src/app 

So I'm mounting the "api" directory of the host Ubuntu OS into docker container under /usr/src/app.

docker inspect <container ID> shows that the volume is writable

"Destination": "/usr/src/app", "Mode": "rw", "RW": true 

However it is not: I get permission denied when I try to create a directory or edit a file from within the docker container.

I googled for this issue, of course, and I came across a few SELinux issues of CentOS/RHEL, but I'm running Ubuntu 15.10, 64 bit edition, not CentOS.

Answer 1

If your uid on the host (id -u) isn't the same as the uid of the user in the docker container (often "docker") then you can have this problem. You can try:

1. Making the UIDs the same between your user and the user in the docker container.
2. Setting the group permissions on the directory to be writable for a group that both you and docker belong to.
3. You could also use the nuclear option:

chmod a+rwx -R project-dir/

The nuclear option will make your git workspace filthy, which will annoy you greatly, so isn't the best long-term solution. It stops the bleeding tho.

For further understanding the problem, you might find these useful:

1. https://github.com/docker/docker/issues/7906
2. https://github.com/docker/docker/issues/7198

Answer 2

New answer:

This questions seems to have a lot of traffic and there is better solution available now - fixuid, as the name suggests it's a magic executable to change the container user's uid & gid on container startup (using -u somebody:somebody).

For a more in dept explanation see: https://boxboat.com/2017/07/25/fixuid-change-docker-container-uid-gid/

---

Old answer:

As of docker version 1.7 you have the option to mount a host directory with permissions to a container using the :Z or :z flags like so:

docker run -v ./api:/usr/src/app:Z 

• :z - will add permissions to all containers using label 'svirt_sandbox_file_t'
• :Z - will add permissions only to the current container label

As of docker-compose v1.4.0, you can use it in docker compose like this:

volumes:    - ./api:/usr/src/app:Z 

Although I should add I still have some problems with this (see Adding permissions to host directory with docker-compose).

References:

Using Volumes with Docker can Cause Problems with SELinux - http://www.projectatomic.io/blog/2015/06/using-volumes-with-docker-can-cause-problems-with-selinux/

Docker user guide - https://docs.docker.com/engine/userguide/dockervolumes/#volume-labels

Docker-compose release notes for v1.4.0 - https://github.com/docker/compose/releases/tag/1.4.0