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_11 in 7 minutes and 48 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Precheck
passed #24416
amd64 docker linux
precheck:debian:sid:amd64

00:00:12

 
  Build
passed #24417
amd64 docker linux
build:centos:centos6:amd64

00:02:45

passed #24418
amd64 docker linux
build:centos:centos7:amd64

00:02:53

passed #24419
amd64 docker linux
build:debian:sid:amd64

00:02:23

passed #24420
i386 docker linux
build:debian:sid:i386

00:02:12

 
  Test
passed #24425
amd64 docker linux
systemtest:centos:centos6:amd64

00:04:29

passed #24426
amd64 docker linux
systemtest:centos:centos7:amd64

00:04:29

passed #24427
amd64 docker linux
systemtest:debian:sid:amd64

00:04:33

passed #24428
i386 docker linux
systemtest:debian:sid:i386

00:04:40

passed #24421
amd64 docker linux
unittest:centos:centos6:amd64

00:01:18

passed #24422
amd64 docker linux
unittest:centos:centos7:amd64

00:01:22

passed #24423
amd64 docker linux
unittest:debian:sid:amd64

00:01:16

passed #24424
i386 docker linux
unittest:debian:sid:i386

00:01:05