Make the Docker build caching more robust

And actually reuse previously built images in CI

Use alpine image for varnish

Fix NelmioCors regex

Fix build caching for the api Dockerfile

Use tmpfs for varnish working directory

Remove useless dependency to core

Improve OPcache settings

Use a single Dockerfile for the API images

Better markup

* Add COC file, remove some files from generated archives

* Remove the code of conduct's content, and add a link to the website

Better eol conversion in .gitattributes

Don't use sh to run command

Since it is no longer necessary...

feature: add .env.dist

Remove usage of obsolete SYMFONY_SKIP_REGISTRATION

Changed var/logs to var/log

To be consistant with sf4, the "logs" folder has been renamed to "log"

Push Docker images to container registry