Docker CLI should use mount API for --tmpfs

[thaJeztah]

This is a follow-up to moby/moby#35467 (comment)

The --tmpfs option adds tmpfs mounts to a container. This option is currently handled daemon-side, where mounts specified through --tmpfs are merged with the default mounts, mounts from -v / --volume, and mounts specified through --mount.

We should change the CLI to use the Mount API

• The mount API was added in API 1.25 (Docker 1.13), added through Add new HostConfig field, Mounts. moby/moby#22373, api: add TypeTmpfs to api/types/mount moby/moby#26837, so can be used on any supported version of Docker (both CE and EE). For older daemons/API versions, the CLI should still use the old options.
• Effectively, --tmpfs becomes an "alias" for --mount type=tmpfs ....

Question

• Should we do the same for --shm-size?
• If we do; where are the defaults set? i.e., we likely don't want the CLI to be aware of any defaults set by the daemon for /dev/shm; how can we provide a custom --shm-size, but keep other defaults as-is?

ping @kolyshkin @cpuguy83 @AkihiroSuda PTAL

[cpuguy83]

How come --shm-size is affected here?

[thaJeztah]

That option sets the size for the default /dev/shm mount; Also wondering if the "use a custom --mount type=tmpfs,target=/dev/shm... approach should be the canonical way to do so.