[Makefile] add 'rebuild-back' and 'rebuild-front'
[Makefile] add 'rebuild-back' and 'rebuild-front'

Those commands can be used to locally rebuild the docker images after
their dependencies are changed.

Fix #189

befair/gasistafelice,michelesr/gasistafelice,befair/gasistafelice,befair/gasistafelice,michelesr/gasistafelice,michelesr/gasistafelice,befair/gasistafelice,michelesr/gasistafelice