DB Testing: Updated supported DB list, added readme

Also fixed potentally flaky GIf test to be more accurate with single
frame GIF images.

dev/docker/db-testing/readme.md

@@ -0,0 +1,32 @@
+# Database Testing Suite
+
+This docker setup is designed to run BookStack's test suite against each major database version we support
+across MySQL and MariaDB to ensure compatibility and highlight any potential issues before a release.
+This is a fairly slow and heavy process, so is designed to just be run manually before a release which
+makes changes to the database schema, or a release which makes significant changes to database queries.
+
+### Running
+
+Everything is ran via the `run.sh` script. This will:
+
+- Optionally, accept a branch of BookStack to use for testing.
+- Build the docker image from the `Dockerfile`.
+  - This will include a built-in copy of the chosen BookStack branch. 
+- Cycle through each major supported database version:
+  - Migrate and seed the database.
+  - Run the full PHP test suite.
+
+If there's a failure for a database version, the script will prompt if you'd like to continue or stop testing.
+
+This script should be ran from this `db-testing` directory:
+
+```bash
+# Enter this directory
+cd dev/docker/db-testing
+
+# Runs for the 'development' branch by default
+./run.sh
+
+# Run for a specific branch
+./run.sh v25-11
+```