Add ANI_CLI_WEB_BRANCH build arg

- add ANI_CLI_WEB_BRANCH to the Docker build so ani-cli-web can be cloned from master or another branch
- pass the new build arg through docker-compose
- document the new branch-selection workflow in the README

docker-compose.yaml

@@ -9,6 +9,7 @@ services:
         ANIKOTO_CLI: ${ANIKOTO_CLI:-https://github.com/Slimyslushy/anikoto-cli.git}
         ANIKOTO_CLI_BRANCH: ${ANIKOTO_CLI_BRANCH:-main}
         ANI_CLI_WEB: ${ANI_CLI_WEB:-https://gitea.coreplay.eu/Dymas/ani-cli-web.git}
+        ANI_CLI_WEB_BRANCH: ${ANI_CLI_WEB_BRANCH:-main}
     image: ani-cli-web:latest
     container_name: ani-cli-web
     restart: unless-stopped