Update lithium-plaintext-g.dockerfile (#5519)

frameworks/C++/lithium/lithium-plaintext-g.dockerfile

@@ -10,6 +10,6 @@ ENV COMMIT=7d5ffc3e3439dc5657348adef7f805bcb74beddd
 RUN wget https://raw.githubusercontent.com/matt-42/lithium/$COMMIT/single_headers/lithium_mysql.hh
 RUN wget https://raw.githubusercontent.com/matt-42/lithium/$COMMIT/single_headers/lithium_http_backend.hh
 
-RUN g++ -g -DTFB_MYSQL -std=c++17 ./lithium.cc -I /usr/include/mariadb -lpthread -lmariadbclient -lboost_context -o /lithium_tbf
+RUN g++ -O3 -g -DTFB_MYSQL -std=c++17 ./lithium.cc -I /usr/include/mariadb -lpthread -lmariadbclient -lboost_context -o /lithium_tbf
 
 CMD gdb -batch -x ./gdb.cmds --args /lithium_tbf tfb-database 8080