feat: add support for rsync (needed by image subdirs)

Procfile

@@ -2,3 +2,4 @@ docs: make watch-docs-internal
 slides: make watch-slides-internal
 toc: make watch-tocupdate-internal
 autoslide: make watch-autoslide-internal
+# proxy: caddy

docker/Dockerfile

@@ -9,7 +9,7 @@ RUN apt-get update \
 
 # Tools for running the common parts
 RUN apt-get update \
- && apt-get install -y --no-install-recommends make build-essential inotify-tools gosu unzip curl \
+ && apt-get install -y --no-install-recommends make build-essential inotify-tools gosu unzip curl rsync \
  && apt-get clean \
  && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* \
  && truncate -s 0 /var/log/*log