WIP: Try to add DABUILD_REUSE

Hi,

Here is a first attempt for have container (or image) reuse (#61).

Although it is working there are some considerations:

• It creates a new image with 'reuse' tag based on first version/arch used. So currently it does not support multiple concurrent versions/arch. Is this needed? If not, should it check if version or arch changed so current 'reuse' image needs to be destroyed?
• It doesn't remove never the 'reuse' image. Should it be removed when abuild sucess? Maybe when run some 'dabuild clean'?
• I had to disable 'set -e' because the 'reuse' image has to be created after 'docker run' command (and it could fail). Is there any problem with it?