ENH: move LINKAHEAD_IMAGE default value to docker-compose

linkahead/.env

 CERTIFICATES_KEY_PASSWORD=Password
-LINKAHEAD_IMAGE=gitlab.indiscale.com:5050/caosdb/src/caosdb-deploy:dev_F_f-doip_P_f-string-ids
 DOIP_SERVICE_ID=TestService
 DNS_HOST_NAME=localhost

linkahead/docker-compose.yml

@@ -7,7 +7,7 @@ services:
     networks:
       - caosnet
   server:
-    image: "$LINKAHEAD_IMAGE"
+    image: "${LINKAHEAD_IMAGE:-gitlab.indiscale.com:5050/caosdb/src/caosdb-deploy:dev_F_f-doip_P_f-string-ids}"
     user: 999:999
     depends_on:
       - sqldb