some cleanup

As suggested by @glensc

docker/Dockerfile

@@ -2,16 +2,13 @@ FROM php:7.4-apache
 
 ARG BRANCH=stable
 
-COPY ./entrypoint.sh /usr/local/sbin/entrypoint
-RUN chmod +x /usr/local/sbin/entrypoint
+COPY --chmod=755 ./entrypoint.sh /usr/local/sbin/entrypoint
 
-# prepare overlay directories
-# https://stackoverflow.com/a/33275168/172068
-RUN ["/bin/bash", "-c", "mkdir -p /overlay/{original,storage/{dokuwiki,work}}"]
 VOLUME /overlay/storage
 
 # setup DokuWiki
-RUN cd /overlay/original && \
+RUN mkdir -p /overlay/original && \
+    cd /overlay/original && \
     curl -L http://github.com/splitbrain/dokuwiki/tarball/$BRANCH | \
     tar --strip-components=1 -xzvf - && \
     chown -R www-data:www-data .

docker/entrypoint.sh

@@ -1,9 +1,11 @@
 #!/bin/bash
 set -e
 
+# prepare overlay directories
+# https://stackoverflow.com/a/33275168/172068
 mkdir -p /overlay/storage/{dokuwiki,work}
 mount -t overlay \
       -o lowerdir=/overlay/original,upperdir=/overlay/storage/dokuwiki,workdir=/overlay/storage/work \
          overlay /var/www/html
 
-docker-php-entrypoint apache2-foreground
+exec docker-php-entrypoint apache2-foreground