Improved the script from https://jpetazzo.github.io/2015/01/13/docker-mount-dynamic-volumes/ to work with more scenarios. Save this file in the same directory as nsenter (https://github.com/jpetazzo/nsenter) and chmod +x it. This allows you to mount a host path on a running container. You may have to adjust the ownership of the mounted directory…

docker-mount.sh

#!/bin/bash -e

# Based on https://jpetazzo.github.io/2015/01/13/docker-mount-dynamic-volumes/

SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"

[[ $# -eq 3 ]] || {
    echo "Usage: $0 <container_name_or_id> <host_path> <container_path>" 1>&2
    exit 1
}

CONTAINER=$1
HOSTPATH=$2
CONTPATH=$3

REALPATH=$(readlink --canonicalize $HOSTPATH)
FILESYS=$(df -P $REALPATH | tail -n 1 | awk '{print $6}')

while read DEV MOUNT FSTYPE JUNK; do
    [ $MOUNT = $FILESYS ] && break
done < /proc/mounts
[ $MOUNT = $FILESYS ] # Sanity check!

while read A B C SUBROOT MOUNT JUNK; do
    [ $MOUNT = $FILESYS ] && break
done < /proc/self/mountinfo
[ $MOUNT = $FILESYS ] # Moar sanity check!

SUBPATH=$(echo $REALPATH | sed s,^$FILESYS,,)
DEVDEC=$(printf "%d %d" $(stat --format "0x%t 0x%T" $(readlink -f $DEV)))

$SCRIPT_DIR/docker-enter $CONTAINER mkdir -p $(dirname $DEV)
$SCRIPT_DIR/docker-enter $CONTAINER sh -c \
         "[ -b $DEV ] || mknod --mode 0600 $DEV b $DEVDEC"
$SCRIPT_DIR/docker-enter $CONTAINER mkdir /tmpmnt
$SCRIPT_DIR/docker-enter $CONTAINER mount -t $FSTYPE $DEV /tmpmnt
$SCRIPT_DIR/docker-enter $CONTAINER mkdir -p $CONTPATH
$SCRIPT_DIR/docker-enter $CONTAINER mount -o bind /tmpmnt/$SUBROOT/$SUBPATH $CONTPATH
$SCRIPT_DIR/docker-enter $CONTAINER umount /tmpmnt
$SCRIPT_DIR/docker-enter $CONTAINER rmdir /tmpmnt