fix: default to concurrency 1 in gitlab-ci-local

This PR makes concurrency of 1 the default for gitlab-ci-local. It prevents running 6 container builds simultaneously, when not intended. This can be overridden on the cli.

.gitlab-ci-local-env

 PRIVILEGED=1
+CONCURRENCY=1
 ULIMIT=8000:16000
 VOLUME=certs:/certs/client
-VARIABLE="DOCKER_TLS_CERTDIR=/certs"
\ No newline at end of file
+VARIABLE="DOCKER_TLS_CERTDIR=/certs"