Add info on Docker tag support

docker/README.md

@@ -16,3 +16,30 @@ Docker support for [`dependabot-core`][core-repo].
    ```
 
 [core-repo]: https://github.com/dependabot/dependabot-core
+
+### Supported tag schemas
+
+Dependabot supports updates for Docker tags that use semver versioning, dates, and build numbers.
+The Docker tag class is located at:
+https://github.com/dependabot/dependabot-core/blob/main/docker/lib/dependabot/docker/tag.rb
+
+#### Semver
+
+Dependabot will attempt to parse a semver version from a tag and will only update it to a tag with a matching prefix and suffix. 
+
+As an example, `base-12.5.1` and `base-12.5.1-golden` would be parsed as `<prefix>-<version>` and `<prefix>-<version>-<suffix>` respectively.
+
+That means for `base-12.5.1` only another `<prefix>-<version>` tag would be a viable update, and for `base-12.5.1-golden`, only another `<prefix>-<version>-<suffix>` tag would be viable. The exception to this is if the suffix is a SHA, in which case it does not get compared and only the `<prefix-<version>` parts are considered in finding a viable tag.
+
+#### Dates
+
+Dependabot will parse dates in the `yyyy-mm`, `yyyy-mm-dd` formats (or with `.` instead of `-`) and update tags to the latest date. 
+
+As an example, `2024-01` will get updated to `2024-02` and `2024.01.29` will get updated to `2024.03.15`.
+
+#### Build numbers
+
+Dependabot will recognize build numbers and will update to the highest build number available.
+
+As an example, `21-ea-32`, `22-ea-7`, and `22-ea-jdk-nanoserver-1809` are mapped to `<version>-ea-<build_num>`, `<version>-ea-<build_num>`, and `<version>-ea-jdk-nanoserver-<build_num>` respectively.
+That means only "22-ea-7" will be considered as a viable update candidate for `21-ea-32`, since it's the only one that respects that format.