Change date to be set by predefined variable in nightly pipelines

Resolves #54 (moved).

This feature changes the way that the date is appended to the image tags for scheduled pipelines. I used a predefined CI/CD variable (CI_PIPELINE_CREATED_AT), which is originally in ISO 8601 format, then stripped the unneeded characters with substrings and tr. This solves the issue of two jobs in the same pipeline having different dates due to date being ran for each job individually, as the predefined variable will stay the same regardless of the job.

Example: 2022-01-31T16:47:55Z --> 220131