feat: support to import an image with image name

[suzuki-shunsuke]

Close #156

• change docker_image's ID from ${imageID}${imageName} to ${imageName}
• support to import docker_image with image name

e.g.

$ terraform import docker_image.alpine alpine:3.13.0

What's appropriate docker_image's ID?

• image ID?
• image name?
• image ID + image name?

I considered about it, and concluded that image name is best.

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[mavogel]

related to #74 . Are you still on this @suzuki-shunsuke ?

[suzuki-shunsuke]

Sorry but I forget the status of this issue.
Probably I kept this pull request draft due to any problem, but I can't remember it.

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