Docker: Add arg for the Etherpad directory

This makes it possible for users to match the directory inside the
container to the host system (for convenience or to avoid confusion).

Dockerfile

@@ -52,7 +52,8 @@ RUN groupadd --system ${EP_GID:+--gid "${EP_GID}" --non-unique} etherpad && \
         ${EP_HOME:+--home-dir "${EP_HOME}"} --create-home \
         ${EP_SHELL:+--shell "${EP_SHELL}"} etherpad
 
-RUN mkdir /opt/etherpad-lite && chown etherpad:etherpad /opt/etherpad-lite
+ARG EP_DIR=/opt/etherpad-lite
+RUN mkdir -p "${EP_DIR}" && chown etherpad:etherpad "${EP_DIR}"
 
 # install abiword for DOC/PDF/ODT export
 RUN [ -z "${INSTALL_ABIWORD}" ] || (apt update && apt -y install abiword && apt clean && rm -rf /var/lib/apt/lists/*)
@@ -63,7 +64,7 @@ RUN [ -z "${INSTALL_SOFFICE}" ] || (apt update && mkdir -p /usr/share/man/man1 &
 
 USER etherpad
 
-WORKDIR /opt/etherpad-lite
+WORKDIR "${EP_DIR}"
 
 COPY --chown=etherpad:etherpad ./ ./
 
@@ -78,7 +79,7 @@ RUN src/bin/installDeps.sh && \
 RUN for PLUGIN_NAME in ${ETHERPAD_PLUGINS}; do npm install "${PLUGIN_NAME}" || exit 1; done
 
 # Copy the configuration file.
-COPY --chown=etherpad:etherpad ./settings.json.docker /opt/etherpad-lite/settings.json
+COPY --chown=etherpad:etherpad ./settings.json.docker "${EP_DIR}"/settings.json
 
 # Fix group permissions
 RUN chmod -R g=u .