Fixed bug in utils.mkbuildcontext for BytesIO Dockerfiles
- resetting dockerfile seek position to 0 after dockerfile.getvalue() is called
- prevents traceback on line utils.py line 43
- 'IOError("end of file reached")' in tarfile.py
Should be done for StringIO objects as well
Merge pull request #271 from robertglen/byteIO-seekfix

Fixed bug in utils.mkbuildcontext for BytesIO Dockerfiles