added environment variable to set preferred OCRmyPDF version when using docker image

- e.g. `- OCRMYPDF_VERSION=v11.2.1`
 - default ist `latest`

docker/joex.dockerfile

@@ -9,6 +9,8 @@ FROM ${REPO}:base-binaries-${VERSION} as docspell-base-binaries
 
 FROM ${REPO}:joex-base-${VERSION}
 
+ENV OCRMYPDF_VERSION=latest
+
 COPY --from=docspell-base-binaries /opt/docspell-joex /opt/docspell-joex
 COPY joex-entrypoint.sh /opt/joex-entrypoint.sh