Add a tail-end flag

[martinchaves]

When the tail-end flag is on the stdout and stderr files are followed starting at end of file instead of at the start.

This flag is equivalent to tail -n 0 ...

[tpetry]

This is really usefull if you restart a container because at the moment dockerize will print all old logging information on stdout/stderr again.

[powerman]

This is already implemented and actually a default behaviour in fork https://github.com/powerman/dockerize