Add solr group variable

Dockerfile

@@ -10,19 +10,20 @@ ENV SOLR_DATA_DIR=/index \
     SOLR_CORE=myth \
     SOLR_ROOT=/opt/solr \
     SOLR_HOME=/opt/solr/server/solr \
-    SOLR_USER=solr
+    SOLR_USER=solr \
+    SOLR_GROUP=solr
 
 USER root
 
 RUN mkdir /index && \
-    chown -R $SOLR_USER:solr $SOLR_DATA_DIR && \
+    chown -R $SOLR_USER:$SOLR_GROUP $SOLR_DATA_DIR && \
     mkdir -p $SOLR_HOME/$SOLR_CORE/ && \
-    chown -R $SOLR_USER:solr $SOLR_HOME/$SOLR_CORE/
+    chown -R $SOLR_USER:$SOLR_GROUP $SOLR_HOME/$SOLR_CORE/
 
 USER $SOLR_USER
 
 COPY --from=indexer /usr/src/app/docs.xml /usr/src/app/
-COPY --chown=$SOLR_USER:solr ./solr-config/* $SOLR_HOME/$SOLR_CORE/
+COPY --chown=$SOLR_USER:$SOLR_GROUP ./solr-config/* $SOLR_HOME/$SOLR_CORE/
 
 RUN touch $SOLR_HOME/$SOLR_CORE/core.properties && \
     solr start && \