Fix and improve Dockerfile
Fix and improve Dockerfile

* Add comment.
* Add missing packages cmake, curl.
* Update bundler (fixes warning).
* Only clone the latest release of travis-build (save time and disk space).
* Remove empty line at end of file.

Signed-off-by: Stefan Weil <eb0877843acc39f8ef6f7269937dee931c372d23@v2201612906741603.powersrv.de>

jbarlow83/tesseract,stweil/tesseract,amitdo/tesseract,amitdo/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,amitdo/tesseract,UB-Mannheim/tesseract,stweil/tesseract,tesseract-ocr/tesseract,jbarlow83/tesseract,tesseract-ocr/tesseract,jbarlow83/tesseract,UB-Mannheim/tesseract,jbarlow83/tesseract,UB-Mannheim/tesseract,jbarlow83/tesseract,tesseract-ocr/tesseract,tesseract-ocr/tesseract,stweil/tesseract,stweil/tesseract,amitdo/tesseract,UB-Mannheim/tesseract,stweil/tesseract,UB-Mannheim/tesseract