How to pre-configure GeoServer in docker image?

I am using GeoServer 2.16.0 docker image build by myself based on kartoza’s dockerfile.

The only pre-configuration is done via environment files, e.g. geoserver.env:

ENABLE_JSONP=true                                                                                                                                         
FOOTPRINTS_DATA_DIR=/opt/footprints_dir
GEOSERVER_ADMIN_PASSWORD=myawesomegeoserver
GEOSERVER_DATA_DIR=/opt/geoserver/data_dir
GEOSERVER_NODE_OPTS="id:$hostname - ($host_ip);background:red;color:white"
GEOWEBCACHE_CACHE_DIR=/opt/geoserver/data_dir/gwc
#JAVA_OPTS="-Duser.country=US -Duser.language=en"
MAX_FILTER_RULES=20
OPTIMIZE_LINE_WIDTH=false

I would like to configure more, e.g. workspaces, stores, but cannot find any according documentation.

Is there any better way than the following?

1. Configuring a running container having the DATA_DIR mounted
2. Create an own image based in kartoza’s and add the DATA_DIR content?

Solution (16.02.2021)

I added the following two lines to my dockerfile:

COPY ./data_dir_init /tmp/data_dir_init
COPY ./init-workspace.sh /scripts

The init-workspace.sh contains the following:

SETUP_LOCKFILE="${GEOSERVER_DATA_DIR}/.init_data_dir.lock"
if [ -f "${SETUP_LOCKFILE}" ]; then
    echo -e "nn Skip GeoServer init - already donenn"
    exit 0
fi

if [ "${DEBUG}" ]; then
    set -e
    set -x
fi

echo -e "nnStarting init of GeoServer data dirnn"

cp --verbose --recursive /tmp/data_dir_init/{styles,workspaces} $GEOSERVER_DATA_DIR
cp --verbose /tmp/data_dir_init/{csw,global,logging,wms,wps}.xml $GEOSERVER_DATA_DIR
mkdir--verbose --parents $GEOSERVER_DATA_DIR/data

if [ ! ${DEBUG} ]; then
    rm --recursive --verbose /tmp/data_dir_init
fi

# Put lock file to make sure data directory is not reinitialized on restart
touch ${SETUP_LOCKFILE}

The folder data_dir_init contains all content from the first session, when I startet an empty geoserver instance and created the base set-up I needed.