[JENKINS-39748] provide Image.exec() to avoid disabling ENTRYPOINT in the image

[hogarth-sv]

There were comments on JENKINS-37987 that the change to overriding ENTRYPOINT on Image.inside() (#63) broke workflows.

On the way to JENKINS-26179 this was a fairly simple change whilst heading towards a Container.exec()

This adds a new Image.exec() {} which behaves identically to Image.inside() {} with the one exception that it does not override ENTRYPOINT so images that rely on ENTRYPOINT not being messed with can still be used.

[mskutin]

Any chance this could be reviewed and merged?

[jglick]

Alternately, JENKINS-40170.