Allow relative paths for mounting volumes to docker containers.

[KartikSoneji]

Description
Trying to mount the current directory with -v .:/source or --mount source=.,target=/source results in an error. Using relative paths causes a new volume to be created instead of mounting the .

Steps to reproduce the issue:

1. Run: docker run -v .:/source alpine OR docker run --mount source=.,target=/source alpine
2. Run:

mkdir source
touch source/test

docker run -it -v ./source:/source alpine
# (in the container's shell)
ls /source

Describe the results you received:

1. The error message:

docker: Error response from daemon: create .: volume name is too short, names should be at least two alphanumeric characters.
See 'docker run --help'.

1. The ls command does not show any files in the /source directory. Instead, a new docker volume is created with the name source.

Describe the results you expected:

1. The current directory to be mounted to the docker container at the path /source.
2. The source folder in the current directory to be mounted to the docker container at the path /source.

Additional information you deem important (e.g. issue happens only occasionally):
The -v and the --mount flags should accept relative paths. Using $(pwd) on Linux and %cd% on Windows is not a pretty solution, and I have to include separate commands for Windows and Linux in my projects.

Output of docker version:

Client: Docker Engine - Community
 Version: 19.03.8
 API version: 1.40
 Go version: go1.12.17
 Git commit: afacb8b
 Built: Wed Mar 11 01:23:10 2020
 OS/Arch: windows/amd64
 Experimental: false

Server: Docker Engine - Community
 Engine:
 Version: 19.03.8
 API version: 1.40 (minimum version 1.12)
 Go version: go1.12.17
 Git commit: afacb8b
 Built: Wed Mar 11 01:29:16 2020
 OS/Arch: linux/amd64
 Experimental: false
 containerd:
 Version: v1.2.13
 GitCommit: 7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc:
 Version: 1.0.0-rc10
 GitCommit: dc9208a3303feef5b3839f4323d9beb36df0a9dd
 docker-init:
 Version: 0.18.0
 GitCommit: fec3683

Output of docker info:

Client:
 Debug Mode: false

Server:
 Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
 Images: 3
 Server Version: 19.03.8
 Storage Driver: overlay2
 Backing Filesystem: <unknown>
 Supports d_type: true
 Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins:
 Volume: local
 Network: bridge host ipvlan macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc version: dc9208a3303feef5b3839f4323d9beb36df0a9dd
 init version: fec3683
 Security Options:
 seccomp
 Profile: default
 Kernel Version: 4.19.76-linuxkit
 Operating System: Docker Desktop
 OSType: linux
 Architecture: x86_64
 CPUs: 2
 Total Memory: 983.4MiB
 Name: docker-desktop
 ID: EGN7:AJIT:6MIR:U54L:PZQX:5JCH:INTW:R3LC:77HP:5IKF:U2YX:Q4WQ
 Docker Root Dir: /var/lib/docker
 Debug Mode: true
 File Descriptors: 34
 Goroutines: 51
 System Time: 2020-05-13T20:43:58.120001272Z
 EventsListeners: 3
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
 127.0.0.0/8
 Live Restore Enabled: false
 Product License: Community Engine

Additional environment details (AWS, VirtualBox, physical, etc.):
Docker Desktop on Windows 10.

[timoteo7]

#1203
#1273

[thaJeztah]

Thanks for linking those; yes, this looks to be a duplicate of #1203. IIRC. the jury was still out on that one, but let me write up a comment there.