Add --shm-size to docker container update

Description

I'd like to easily change the shm-size in a running container by using docker container update. I'm running PostgreSQL inside a container and receiving an error related to the small amount of shared memory available.

Steps to reproduce the issue:

1. Run any container you want to, without passing --shm-size
2. Use some software inside the container to fill the 64MB shared memory available.
3. Done, you're out of shared memory and you can't change it using the docker cli without destroying the container and creating a new one.

Describe the results you received:

I've found two ways of changing the shared memory of a running container:

1. Remount the shm file system of the container: mount -o remount,rw,nosuid,nodev,noexec,relatime,size=256M -t tmpfs /var/lib/docker/containers/<container-id>/mounts/shm. If shm is not found inside this path, try using nsenter as explained in https://github.com/docker/cli/issues/1278#issuecomment-1280877760

2. Change ShmSize inside /var/lib/docker/containers/<container-id>/hostconfig.json and restart container. EDIT: as noted by @dmitrygashnikov: you must stop docker service (service docker stop), then change the hostconfig.json file for this container and start docker again (service docker start).

The second option is the only one which makes the change persistent, but we need to stop all running containers to make the change.

Here is a little script to change /dev/shm (by default to 1GB), using the docker daemon restart method:

#!/bin/bash

set -e
CONTAINER_NAME="$1"
if [ -z "$CONTAINER_NAME" ]; then
        echo "ERROR - Usage: $0 <container-name> [shm-size]"
        exit 1
fi
if [ ! -z "$2" ]; then
        SHM_SIZE="$2"
else
        SHM_SIZE="1073741824"
fi

echotime() {
        echo [$(date "+%Y-%m-%d %H:%M:%S")] $@
}

CONTAINER_ID=$(docker inspect -f '{{ .ID }}' $CONTAINER_NAME)
HOST_CONFIG=/var/lib/docker/containers/$CONTAINER_ID/hostconfig.json

echotime "Stopping docker service"
service docker stop

echotime "Changing container's hostconfig.json"
sed -i 's/"ShmSize":[0-9]\+,/"ShmSize":'$SHM_SIZE',/' $HOST_CONFIG

echotime "Starting docker service"
service docker start

Describe the results you expected:

It would be awesome to change this setting from the command line, using docker container update --shm-size=256M.

Output of docker version:

Client:
 Version:      18.03.1-ce
 API version:  1.37
 Go version:   go1.9.5
 Git commit:   9ee9f40
 Built:        Thu Apr 26 07:17:14 2018
 OS/Arch:      linux/amd64
 Experimental: false
 Orchestrator: swarm

Server:
 Engine:
  Version:      18.03.1-ce
  API version:  1.37 (minimum version 1.12)
  Go version:   go1.9.5
  Git commit:   9ee9f40
  Built:        Thu Apr 26 07:15:24 2018
  OS/Arch:      linux/amd64
  Experimental: false

Start by tracing the docker container update command and its existing option handling; the issue names no implementation files or tests. Check how --shm-size is represented and updated, then verify that the requested command changes a running container without recreating it and preserves the setting.