Use UK formatted date for postDate filter

[jeff-horton-ho-sas]

• Add luxon as an explicit dependency rather than assuming it will be installed as a transitive dependency of 11ty
• The display discrepancy is due to the site being built in an environment with an American Locale.
  • It is clearer to specify an explicit date format to remind contributors that the displayed date is only locale specific at build time, and is static in the generated site.
  • Included @LiamMacP 's suggestion to also update the locale and timezone in the Docker image used to build the site.

Code change

I can confirm:

Accessibility considerations

• [X] Please review the accessibility checks for layout changes.

• [X] This change will not change layouts, page structures or anything else that might impact accessibility

[LiamMacP]

I've decided to specify an explicit date format as that expresses the intent more clearly than changing the locale in the Dockerfile.

Is there any side effect from doing both? No issues with this immediate change, but the lower level change should propagate more cleanly. Even at the expense of slight unclarity.

[jeff-horton-ho-sas]

I've decided to specify an explicit date format as that expresses the intent more clearly than changing the locale in the Dockerfile.

Is there any side effect from doing both? No issues with this immediate change, but the lower level change should propagate more cleanly. Even at the expense of slight unclarity.

Good point. I've added localisation to build image.