store and check for updated image hashes when launching

[leonstafford]

after user chooses the php 8 or 7 version for their site, check for updated docker image and refresh/replace existing if out of date

this simplifies evergreen base images of php8 and php7

Q: how to check if fresher image exists?

[leonstafford]

something like docker pull leonstafford/lokl:php8

can run this each time and show message "Checking for updated Lokl php8 image"

[leonstafford]

easy check of URL HEAD and checksum could work just as well with elementor/lokl#26