Fix issues with variable in DOCKER_CERT_PATH

This MR fixes an issue with $DOCKER_CERT_PATH containing the $DOCKER_TLS_CERTDIR variable. We've seen multiple issues with this variable expanding as an empty value. So, we're simply hard coding it instead which appears to have resolved the issue.

Broken pipeline: https://gitlab.developers.cam.ac.uk/uis/devops/digital-admissions/admissions-portal/ap-backend/-/jobs/1053237

Fixed pipeline: https://gitlab.developers.cam.ac.uk/uis/devops/digital-admissions/admissions-portal/ap-backend/-/jobs/1053272