Up to Docker 1.7, docker inspect provided information about volume mounts in Volumes.

Let's assume we've bind mounted directory /tmp/test to a container:

host$ mkdir /tmp/test
host$ echo test-mount > /tmp/test/test
host$ docker run --name test-mount -ti -v /tmp/test:/data:Z fedora:22 cat /data/test
test-mount

We could then get information about the directory being mounted with docker inspect and would see

host$ docker inspect test-mount
[
{
...
    "Volumes": {
        "/data": "/tmp/test"
    },
    "VolumesRW": {
        "/data": true
    },
...
}
]

If we only wanted to output the name of directory being mounted as /data, we would have run

host$ docker inspect --format '{{ index .Volumes "/data" }}' test-mount
/tmp/test

The use of index here was needed because the key /data is not alpha numeric so we couldn't have done '{{ .Volumes./data }}'

With Docker 1.8, the Volumes structure is missing from the docker inspect output. When we run inspect with the original format, we get an error:

host$ docker inspect --format '{{ index .Volumes "/data" }}' test-mount
reflect: call of reflect.Value.Type on zero Value

When we run the same docker run -v /tmp/test:/data:Z with Docker 1.8 and then inspect the container, we will see the format has changed — Volumes are gone and there are Mounts instead, with different structure:

host$ docker inspect test-mount
[
{
...
    "Mounts": [
        {
            "Source": "/tmp/test",
            "Destination": "/data",
            "Mode": "Z",
            "RW": true
        }
    ],
...
}
]

To output the same information as with older versions, the directory being mounted into /data, the format needs to be changed a bit:

host$ docker inspect --format '{{ range .Mounts }}{{ if eq .Destination "/data" }}{{ .Source }}{{ end }}{{ end }}' test-mount
/tmp/test

Since the structure is no longer a map but array, we need to do a little bit more work to cycle through the list with range to find the mount point we are interested in.