Switch to OpenJ9 base image (#360)

README.md

@@ -1015,6 +1015,14 @@ via a `JVM_XX_OPTS` environment variable.
 For some cases, if e.g. after removing mods, it could be necessary to startup minecraft with an additional `-D` parameter like `-Dfml.queryResult=confirm`. To address this you can use the environment variable `JVM_DD_OPTS`, which builds the params from a given list of values separated by space, but without the `-D` prefix. To make things running under systems (e.g. Plesk), which doesn't allow `=` inside values, a `:` (colon) could be used instead. The upper example would look like this:
 `JVM_DD_OPTS=fml.queryResult:confirm`, and will be converted to `-Dfml.queryResult=confirm`. 
 
+The container uses [OpenJ9](https://www.eclipse.org/openj9/docs) and a couple of J9 options are
+simplified by environment variables:
+
+- `-e TUNE_VIRTUALIZED=TRUE` : enables the option to 
+  [optimize for virtualized environments](https://www.eclipse.org/openj9/docs/xtunevirtualized/)
+- `-e TUNE_NURSERY_SIZES=TRUE` : configures nursery sizes where the initial size is 50%
+  of the `MAX_MEMORY` and the max size is 80%.
+
 ### HTTP Proxy
 
 You may configure the use of an HTTP/HTTPS proxy by passing the proxy's URL via the `PROXY`