Feature: When using custom built images, ssdev should recognize wether the Dockerfile has changes in it since the last build and automatically rebuild

[mleutenegger]

This feature would improve project portability, as you would not have to tell other developers to rebuild the host image.

There is some mor research involved to check wether this is possible at all.

A PR must fulfill the following:

• ssdev or docker automatically rebuild the image if the file has changed