Environment variables with multiple '=' are not parsed correctly

[AndreaGiardini]

This pull request provides a failing integration test.

Whenever a Dockerfile contains an environment variable (ENV) with multiple = the final image contains an env variable with a stripped-down version of the content.

It would be nice if somebody could point me in the right direction to fix this problem :)

Thanks

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[AndreaGiardini]

I signed it!

[googlebot]

CLAs look good, thanks!

[AndreaGiardini]

A failing test case and the fix have been provided :)

[priyawadhwa]

Thanks for fixing this!