Timestamping tags for dependabot usecases

[ruffsl]

It'd be neat to add/rotate the same datetime pattern to the official ROS image tags as Ubuntu for dependabot:

• 🐳 Bump ubuntu from noble-20240429 to noble-20240530 in /ros2/testing/testing #751

Then folks could automate the version bump and rebuild of their child docker images whenever a new release is detected. Although, perhaps this can be achieved by pinning the tag via the image digest, so maybe not worth it.

- FROM ros:jazzy-20240605 
+ FROM ros:jazzy@sha256:1c445ce6bd87d8bc884fcd40e8a9d8f7efd939e2e4d321c2c9453d1c3e3ad541

[mikaelarguedas]

👍

I was considering two things regarding timestamping (that could also ensure cache bursting and image rebuild on new ROS syncs) .

• Storing sync date in an environment variable e.g. ROS_SYNC_DATE so that users can know from within the environment what version of the ROS stack they are using (something like the rosdistro tags stripped from the distro name: rolling/2024-05-24 -> 2024-05-24
• Adding the sync date to the tag like done for ubuntu 👍

WDYT ?