Pull fixed version of calibre based on build time

Dockerfile

@@ -8,6 +8,13 @@ ARG DEBIAN_FRONTEND="noninteractive"
 LABEL build_version="Linuxserver.io version:- ${VERSION} Build-date:- ${BUILD_DATE}"
 LABEL maintainer="chbmb"
 
+RUN \
+  mkdir -p \
+    /root-layer && \
+  CALIBRE_RELEASE=$(curl -sX GET "https://api.github.com/repos/kovidgoyal/calibre/releases/latest" \
+  | awk '/tag_name/{print $4;exit}' FS='[""]' | sed 's/^v//g') && \
+  echo $CALIBRE_RELEASE > /root-layer/CALIBRE_RELEASE
+
 # copy local files
 COPY root/ /root-layer/