Change pull strategy to always pull on page load

[ScottG489]

If the backing docker image changes, it's never updated since we use pull=absent.

To make sure we use the most up to date image, we should always have the backend pull once, probably on page load, and then only pull if it's absent until the page is reloaded again.

[ScottG489]

To clarify, we still should use absent because if the backing image is removed while the page is loaded, it still needs to re-pull.

[ScottG489]

Rethinking this, this will cause initial page loads to be slower, and this is generally when we want things to be faster. When it refreshes every 30 mins we probably don't care that it takes a little longer.

Perhaps we should do an absent pull on initial page load, and then refreshes will do an always.

[ScottG489]

Fixed by cacd900.