Feature Request: Support zip or flat files for context import/export

[kinghuang]

Description

In CLI 19.03, docker contexts can be imported/exported via tar files. I'd like to be able to generate contexts from Terraform configs, but the archive_file data source only supports outputting zip files.

It would be helpful if docker context import/export supported zip files for Docker contexts. Or, a flat file config like kubeconfigs (basically the meta.json with certs embedded).

Steps to reproduce the issue:

1. Create a Docker context as a zip archive.
2. Attempt to import it with docker context import …

Describe the results you received:

archive/tar: invalid tar header

Describe the results you expected:

example-context
Successfully imported context "example-context"

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Client: Docker Engine - Community
 Version:           19.03.0-beta3
 API version:       1.40
 Go version:        go1.12.4
 Git commit:        c55e026
 Built:             Thu Apr 25 19:05:38 2019
 OS/Arch:           darwin/amd64
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          19.03.0-beta3
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.12.4
  Git commit:       c55e026
  Built:            Thu Apr 25 19:13:00 2019
  OS/Arch:          linux/amd64
  Experimental:     true
 containerd:
  Version:          v1.2.6
  GitCommit:        894b81a4b802e4eb2a91d1ce216b8817763c29fb
 runc:
  Version:          1.0.0-rc7+dev
  GitCommit:        029124da7af7360afa781a0234d1b083550f797c
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683

Output of docker info:

N/A

Additional environment details (AWS, VirtualBox, physical, etc.):

[kinghuang]

Zip file support for docker context import was merged in #1895, and backported to 19.03 in #1912. I'm able to import zip files in 19.03.0-rc2.