Add Docker branch selector for ani-cli-web

Add an  build argument to the Docker image so the cloned ani-cli-web source can come from a selected branch, and document the new build option in the README.

Dockerfile

@@ -3,12 +3,14 @@ FROM python:3.12-slim
 ARG ANI_CLI=https://github.com/pystardust/ani-cli.git
 ARG ANI_CLI_BRANCH=master
 ARG ANI_CLI_WEB=https://gitea.coreplay.eu/Dymas/ani-cli-web.git
+ARG ANI_CLI_WEB_BRANCH=main
 ARG YT_DLP_RELEASE_URL=https://github.com/yt-dlp/yt-dlp/releases/latest/download/yt-dlp
 
 ENV DEBIAN_FRONTEND=noninteractive \
     ANI_CLI_REPO=${ANI_CLI} \
     ANI_CLI_BRANCH=${ANI_CLI_BRANCH} \
     ANI_CLI_WEB_REPO=${ANI_CLI_WEB} \
+    ANI_CLI_WEB_BRANCH=${ANI_CLI_WEB_BRANCH} \
     ANI_CLI_BIN=ani-cli \
     ANI_CLI_DOWNLOAD_DIR=/downloads \
     ANI_CLI_WEB_HOST=0.0.0.0 \
@@ -46,7 +48,7 @@ RUN curl -fL "${YT_DLP_RELEASE_URL}" -o /usr/local/bin/yt-dlp \
     && chmod 0755 /usr/local/bin/yt-dlp \
     && yt-dlp --version
 
-RUN git clone --depth 1 "${ANI_CLI_WEB}" /app \
+RUN git clone --depth 1 --branch "${ANI_CLI_WEB_BRANCH}" "${ANI_CLI_WEB}" /app \
     && rm -rf /app/.git \
     && rm -f \
         /app/.dockerignore \