Minor fixes and improvements on instructor images and instructions

Author: rafabene

instructor/docker-compose.yml

@@ -23,7 +23,7 @@ nexus:
     - data-volume
   restart: always
 postgresqlgitlab:
-  image: quay.io/sameersbn/postgresql:9.4-5
+  image: sameersbn/postgresql:9.4-5
   environment:
     - DB_USER=gitlab
     - DB_PASS=password
@@ -37,7 +37,7 @@ redisgitlab:
     - /srv/docker/gitlab/redis:/var/lib/redis
   restart: always
 gitlab:
-  image: quay.io/sameersbn/gitlab:8.0.5
+  image: sameersbn/gitlab:8.0.5
   links:
     - redisgitlab:redisio
     - postgresqlgitlab:postgresql

instructor/readme.adoc

@@ -209,7 +209,7 @@ ping classroom.example.com and verify it works.
 
 [source, text]
 ----
-docker-machine create --driver=virtualbox --virtualbox-memory=4096 --virtualbox-boot2docker-url=https://github.com/boot2docker/boot2docker/releases/download/v1.9.0/boot2docker.iso --engine-insecure-registry=classroom.example.com:5000 instructor-machine
+docker-machine create --driver=virtualbox --virtualbox-memory=4096 --virtualbox-cpu-count=4 --virtualbox-boot2docker-url=https://github.com/boot2docker/boot2docker/releases/download/v1.9.0/boot2docker.iso --engine-insecure-registry=classroom.example.com:5000 instructor-machine
 ----