feat: add dynamic image tags based on service

[Serafin-dev]

Description

This PR aims to add a feature to set Open edX images tags dinamically based on service. It also helps to standarize the way tags are built for different organizations.
The variable needed to enable this feature should be defined at the strain config file as:

Element	Description	Required	Default Value
DOCKER_REGISTRY	The Docker registry where images are stored.	Optional	docker.io
REPO_BASE	The base of the Docker repository for the service.	Optional	tutor-openedx/{service}-tutor
TUTOR_VERSION	The version of Tutor used.	Required	-
TIMESTAMP	a timestamp with the format YYYY-MM-DD-HH-mm	-	Automatically generated if the feature is enabled.

How to test

• Add this key-value to your strain config.yml file:

ENABLE_DYNAMIC_IMAGE_TAG: true

• Build an image for any service

Optional

The default value for REPO_BASE is also built dinamically if not provided.
If the default does not work for your case just define a custom repository path:

REPO_BASE: my-registry/repo-base

Other considerations

There is also a possiblity to just ask for dispatch to pass a complete BASE_TAG that includes all the path but the timestamp. e.g:

tutor config save --set "${DOCKER_IMAGE}=${BASE_TAG}-${TIMESTAMP}"
# or keeping TUTOR_VERSION 
tutor config save --set "${DOCKER_IMAGE}=${REGISTRY_BASE}:${TUTOR_VERSION#v}-${TIMESTAMP}"

The one that I am proposing consider reusing variables that the workflow already uses.