Add bat-server.dockerfile

Pending a Compose file defining the required volumes.

This is a first step towards dockerizing bat, which will prevent my
messing with the local instance's cache.

Pain point: when I'm in the middle of making big changes to
custom16.tmTheme, without re-building the bat cache yet. Then the
"restore user configuration" step in regression scripts updates the
cache with those wip changes.

As a rule of thumb, messing with the user's (contributor's?) local
setup is a bad thing.

Credit for the CURL options:
https://veducate.co.uk/download-releases-from-github-using-curl-and-wget/

tests/bat-server.dockerfile

@@ -0,0 +1,12 @@
+# usage: through the docker-compose service
+
+FROM curlimages/curl:latest as fetch-pkg
+LABEL keep=false
+ARG BAT_VERSION=0.21.0
+RUN curl -LJ --output /tmp/bat.deb \
+    https://github.com/sharkdp/bat/releases/download/v$BAT_VERSION/bat_${BAT_VERSION}_amd64.deb
+
+FROM debian:bullseye-slim
+COPY --from=fetch-pkg /tmp/bat.deb /tmp
+RUN dpkg --install /tmp/bat.deb
+CMD sleep infinity