Allowed override of parallel jobs in docker build.
Allowed override of parallel jobs in docker build.

Use JOBS env variable to control `make -j` parameter.
By default `JOBS=6`.

Review: https://reviews.apache.org/r/63315/

abudnik/mesos,andschwa/mesos,kaysoky/mesos,andschwa/mesos,Gilbert88/mesos,andschwa/mesos,andschwa/mesos,abudnik/mesos,Gilbert88/mesos,gsantovena/mesos,chhsia0/mesos,chhsia0/mesos,kaysoky/mesos,reneploetz/mesos,dforsyth/mesos,asamerh4/mesos,asamerh4/mesos,Gilbert88/mesos,abudnik/mesos,jpeach/mesos,chhsia0/mesos,reneploetz/mesos,gsantovena/mesos,jpeach/mesos,Gilbert88/mesos,jpeach/mesos,asamerh4/mesos,gsantovena/mesos,dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,kaysoky/mesos,kaysoky/mesos,abudnik/mesos,reneploetz/mesos,andschwa/mesos,chhsia0/mesos,asamerh4/mesos,gsantovena/mesos,kaysoky/mesos,Gilbert88/mesos,Gilbert88/mesos,andschwa/mesos,kaysoky/mesos,chhsia0/mesos,chhsia0/mesos,asamerh4/mesos,dforsyth/mesos,reneploetz/mesos,gsantovena/mesos,abudnik/mesos,kaysoky/mesos,dforsyth/mesos,dforsyth/mesos,dforsyth/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,jpeach/mesos,asamerh4/mesos,andschwa/mesos,gsantovena/mesos,dforsyth/mesos,asamerh4/mesos,jpeach/mesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,chhsia0/mesos,jpeach/mesos