Use env vars passed via overrides in output image

[eharris128]

Fixes-###

What

• Passing --env FOO=BAR with --image-overrides=all || --image-overrides=env when invoking docker slim's build command, should result in the output image having an environmental variable FOO=BAR.
• Extend documentation for flags that can be modified via --image-overrides to better reflect the difference between flags targeting runtime versus generated images.

Why

• Propagate env var changes to output images.

How Tested

• Build arbitrary image with an env var FOO=bar. Could instantiate this in your image's Dockerfile.
• Run docker slim build against this image with the following two flags--env FOO=notbar --image-overrides=env
• Inspect the generated image (docker inspect || docker exec || docker slim xray).
• You should see FOO=notbar

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