More Dockerfile updates:  * use Erlang Solutions deb package to get Erlang 18  * use rebar 2.6.0
More Dockerfile updates:
 * use Erlang Solutions deb package to get Erlang 18
 * use rebar 2.6.0

yurrriq/lfe,arpunk/lfe,jjl/lfe,yurrriq/lfe,oubiwann/lfe,bsmr-erlang/lfe,oubiwann/lfe,arpunk/lfe,bsmr-erlang/lfe,jjl/lfe