Switch to alpine and update build process

[rinkp]

Warning

This release contains several potentially breaking changes, see the section below

Faster build process

• Separately building ARM/AMD and then combining
• Various cached steps as part of the docker build process

Smaller image

• Switch from apache default image to alpine with apache
• Only include DejaVu family fonts (needed for export to pdf)
• Uncompressed image size is reduced from 665 MB to 111 MB (compressed: 232 -> 36 MB)

New features

• Introduce optional environment variables OST_HELPDESK_URL and OST_HELPDESK_ONLINE to control online status of the helpdesk interface through environment variables.
• Automatically configure filesystem attachments

Breaking changes

• When running this release, your osTicket installation will automatically store your attachments on the filesystem. The system will not start if the path to the attachments is not a volume to prevent data loss. Be careful if this is an unnamed volume. To disable attachments on the filesystem, set OST_PLUGINS_STORAGEFS_PATH=''
• If you used this image as a base image, please note that the osTicket installation can now be found in /var/www/localhost/htdocs (previously /var/www/html/). For compatibility reasons, a Dockerfile, Dockerfile-legacy, is present to build the old image.