feat: set db type

compose.postgresql.yml

@@ -2,6 +2,7 @@ version: "3.8"
 services:
   app:
     environment:
+      - CMD_DB_TYPE=postgres
       - CMD_DB_NAME=codimd
       - CMD_DB_USER=codimd
       - CMD_DB_HOST=db

entrypoint.sh.tmpl

@@ -43,8 +43,7 @@ main() {
 main
 
 if [ -z "${CMD_DB_URL:-""}" ] ; then
-  # TODO: support databases other than postgres and sqlite
-  export CMD_DB_URL="${postgres://$CMD_DB_USER:$CMD_DB_PASSWORD@$CMD_DB_HOST:5432/$CMD_DB_NAME}"
+  export CMD_DB_URL="${CMD_DB_TYPE}://$CMD_DB_USER:$CMD_DB_PASSWORD@$CMD_DB_HOST:5432/$CMD_DB_NAME}"
 fi
 
 # 3wc: `source /docker-entrypoint.sh -e` to load CMD_DB_URL for CLI scripts