Add CentOS 6/7 to CI

Modify .gitlab-ci.yml so that every CI pipeline also builds and tests
BIND on CentOS versions 6 and 7.  Use --disable-warn-error on CentOS 6
since it uses GCC 4.4.7 which suffers from bugs causing bogus warnings
to be generated, e.g.:

    sigs_test.c: In function 'compare_tuples':
    sigs_test.c:75: warning: declaration of 'index' shadows a global declaration
    /usr/include/string.h:489: warning: shadowed declaration is here
    sigs_test.c: In function 'updatesigs_test':
    sigs_test.c:193: warning: declaration of 'index' shadows a global declaration
    /usr/include/string.h:489: warning: shadowed declaration is here

(cherry picked from commit f0966d14)
13 jobs for 365-add-centos-to-ci-v9_12 in 8 minutes and 16 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Precheck
passed #24429
amd64 docker linux
precheck:debian:sid:amd64

00:00:12

 
  Build
passed #24430
amd64 docker linux
build:centos:centos6:amd64

00:02:45

passed #24431
amd64 docker linux
build:centos:centos7:amd64

00:02:53

passed #24432
amd64 docker linux
build:debian:sid:amd64

00:02:07

passed #24433
i386 docker linux
build:debian:sid:i386

00:02:12

 
  Test
passed #24438
amd64 docker linux
systemtest:centos:centos6:amd64

00:04:57

passed #24439
amd64 docker linux
systemtest:centos:centos7:amd64

00:04:52

passed #24440
amd64 docker linux
systemtest:debian:sid:amd64

00:04:56

passed #24441
i386 docker linux
systemtest:debian:sid:i386

00:05:07

passed #24434
amd64 docker linux
unittest:centos:centos6:amd64

00:01:08

passed #24435
amd64 docker linux
unittest:centos:centos7:amd64

00:01:04

passed #24436
amd64 docker linux
unittest:debian:sid:amd64

00:00:57

passed #24437
i386 docker linux
unittest:debian:sid:i386

00:01:01