Docker: Install `git` and `ca-certificates` packages

This makes it possible to install plugins from Git repositories.

Dockerfile

@@ -57,11 +57,12 @@ RUN mkdir -p "${EP_DIR}" && chown etherpad:etherpad "${EP_DIR}"
 
 # the mkdir is needed for configuration of openjdk-11-jre-headless, see
 # https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=863199
-RUN [ -n "${INSTALL_ABIWORD}${INSTALL_SOFFICE}" ] || exit 0; \
-    export DEBIAN_FRONTEND=noninteractive; \
+RUN export DEBIAN_FRONTEND=noninteractive; \
     mkdir -p /usr/share/man/man1 && \
     apt-get -qq update && \
     apt-get -qq --no-install-recommends install \
+        ca-certificates \
+        git \
         ${INSTALL_ABIWORD:+abiword} \
         ${INSTALL_SOFFICE:+libreoffice} \
         && \