Copy files to Docker volume

copy-to-docker-volume.sh

# Usage: copy-to-docker-volume SRC_PATH DEST_VOLUME_NAME [DEST_PATH]
copy-to-docker-volume() {
  SRC_PATH=$1
  DEST_VOLUME_NAME=$2
  DEST_PATH="${3:-}"
  # create smallest Docker image possible
  echo -e 'FROM scratch\nLABEL empty=""' | docker build -t empty -
  # create temporary container to be able to mount volume
  CONTAINER_ID=$(docker container create -v my-volume:/data empty cmd)
  # copy files to volume
  docker cp "${SRC_PATH}" "${CONTAINER_ID}":"/data/${DEST_PATH}"
  # remove temporary container
  docker rm "${CONTAINER_ID}"
}

example-copy-to-path.sh

# create volume as destination
docker volume create my-volume
# create directory to copy
mkdir my-dir
echo "hello file1" > my-dir/my-file-1
# copy directory to volume
copy-to-docker-volume my-dir my-volume
# list directory on volume
docker run --rm -it -v my-volume:/data busybox ls -la /data/my-dir
# show file content on volume
docker run --rm -it -v my-volume:/data busybox cat /data/my-dir/my-file-1

# create another fil to copy
echo "hello file2" > my-file-2
# copy file to directory on volume
copy-to-docker-volume my-file-2 my-volume my-dir
# list (updated) directory on volume
docker run --rm -it -v my-volume:/data busybox ls -la /data/my-dir
# check volume content
docker run --rm -it -v my-volume:/data busybox cat /data/my-dir/my-file-2

See my answer to the question What is the right way to add data to an existing named volume in Docker?