Use centos6-test image in Concourse task files

Also updated docker README to reference new build image.
Co-authored-by: NDavid Sharp <dsharp@pivotal.io>
Co-authored-by: NKaren Huddleston <khuddleston@pivotal.io>

README.docker.md

@@ -21,7 +21,7 @@ To make Greenplum accessible outside of the docker container use the -p switch (
 This will proxy connections / requests made to the `<port_on_local_machine>` across to `<greenplum_port_in_docker>` which will allow Greenplum within docker to receive them.
 
 ```bash
-docker run -t -v ~/workspace/gpdb:/home/gpadmin/gpdb_src --privileged --security-opt seccomp:unconfined -i pivotaldata/centos-gpdb-dev:7-gcc6.2-llvm3.7 bash
+docker run -t -v ~/workspace/gpdb:/home/gpadmin/gpdb_src --privileged --security-opt seccomp:unconfined -i pivotaldata/centos7-build:gpdb6-latest bash
 ```
 
 ## Set up Container Dependencies and the gpadmin User

concourse/tasks/gpcheckcloud_tests_gpcloud.yml

@@ -2,8 +2,8 @@ platform: linux
 image_resource:
   type: docker-image
   source:
-    repository: pivotaldata/centos-gpdb-dev
-    tag: '6-gcc6.2-llvm3.7'
+    repository: pivotaldata/centos6-test
+    tag: gpdb6-latest
 inputs:
   - name: gpdb_src
   - name: bin_gpdb

concourse/tasks/regression_tests_gpcloud.yml

@@ -2,8 +2,8 @@ platform: linux
 image_resource:
   type: docker-image
   source:
-    repository: pivotaldata/centos-gpdb-dev
-    tag: '6-gcc6.2-llvm3.7'
+    repository: pivotaldata/centos6-test
+    tag: gpdb6-latest
 inputs:
   - name: gpdb_src
   - name: bin_gpdb

concourse/tasks/regression_tests_gphdfs.yml

@@ -2,7 +2,8 @@ platform: linux
 image_resource:
   type: docker-image
   source:
-    repository: pivotaldata/centos-gpdb-dev
+    repository: pivotaldata/centos6-test
+    tag: gpdb6-latest
     username:
     password:
 inputs:

concourse/tasks/regression_tests_gphdfs_mapr.yml

@@ -2,7 +2,8 @@ platform: linux
 image_resource:
   type: docker-image
   source:
-    repository: pivotaldata/centos-gpdb-dev
+    repository: pivotaldata/centos6-test
+    tag: gpdb6-latest
     username:
     password:
 

concourse/tasks/unit_tests_gpcloud.yml

@@ -2,8 +2,8 @@ platform: linux
 image_resource:
   type: docker-image
   source:
-    repository: pivotaldata/centos-gpdb-dev
-    tag: '6-gcc6.2-llvm3.7'
+    repository: pivotaldata/centos6-test
+    tag: gpdb6-latest
     username:
     password:
 inputs: