Fixed docker buildx errors in CI/CD pipeline:
1. Set DOCKER_TLS_CERTDIR to empty string to disable TLS
(buildx with docker-in-docker doesn't work well with TLS certs)
2. Added fallback to reuse existing builder if creation fails
(docker buildx create ... || docker buildx use ...)
This fixes the error:
'could not create a builder instance with TLS data loaded from environment'
- Change registry from GitLab to Docker Hub (gmcintire/towerops-agent)
- Update authentication to use DOCKERHUB_USERNAME and DOCKERHUB_TOKEN
- Remove vntx runner tags to use GitLab shared runners
