Image based drivers should not include host env vars

[schmichael]

Image based drivers such as Docker often have their own environment variables such as PATH. #2054 (first released in 0.5.2) caused host environment variables, such as PATH, to be added to all image based drivers and override any existing environment variables.

Only exec based drivers should include host environment variables by default. Image based drivers like Docker should not.

The documentation also needs updating.

Original report: https://groups.google.com/d/msg/nomad-tool/uP1tKiOLLmk/cnG_dFZzFQAJ

[github-actions]

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