Add healthcheck to Dockerfile

Dockerfile

@@ -63,6 +63,7 @@ RUN export DEBIAN_FRONTEND=noninteractive; \
     apt-get -qq --no-install-recommends install \
         ca-certificates \
         git \
+        curl \
         ${INSTALL_ABIWORD:+abiword} \
         ${INSTALL_SOFFICE:+libreoffice} \
         && \
@@ -94,5 +95,7 @@ COPY --chown=etherpad:etherpad ./settings.json.docker "${EP_DIR}"/settings.json
 # Fix group permissions
 RUN chmod -R g=u .
 
+HEALTHCHECK --interval=20s --timeout=3s CMD curl -f http://localhost:9001 || exit 1
+
 EXPOSE 9001
 CMD ["node", "src/node/server.js"]