ci/Dockerfile: Warn if ulimit -n is too high when running Docker

The underlying issue does not affect our CI hosts, but is an issue on my
development machine (Arch Linux). In particular, this affects the vanilla
configuration of Docker on systemd, which has effectively no limit:
https://github.com/docker/packaging/blob/11400a3f5a20f2e3eecc3e6347a2ad9ce41278c7/pkg/docker-engine/common/systemd/docker.service#L31

I hope this saves future generations some precious hours of their life.

Author: real-or-random

ci/linux-debian.Dockerfile

@@ -2,6 +2,17 @@ FROM debian:stable-slim
 
 SHELL ["/bin/bash", "-c"]
 
+WORKDIR /root
+
+# A too high maximum number of file descriptors (with the default value
+# inherited from the docker host) can cause issues with some of our tools:
+#  - sanitizers hanging: https://github.com/google/sanitizers/issues/1662 
+#  - valgrind crashing: https://stackoverflow.com/a/75293014
+# This is not be a problem on our CI hosts, but developers who run the image
+# on their machines may run into this (e.g., on Arch Linux), so warn them.
+# (Note that .bashrc is only executed in interactive bash shells.)
+RUN echo 'if [[ $(ulimit -n) -gt 200000 ]]; then echo "WARNING: Very high value reported by \"ulimit -n\". Consider passing \"--ulimit nofile=32768\" to \"docker run\"."; fi' >> /root/.bashrc
+
 RUN dpkg --add-architecture i386 && \
     dpkg --add-architecture s390x && \
     dpkg --add-architecture armhf && \
@@ -24,8 +35,6 @@ RUN apt-get update && apt-get install --no-install-recommends -y \
         gcc-mingw-w64-i686-win32 wine32 \
         python3
 
-WORKDIR /root
-
 # Build and install gcc snapshot
 ARG GCC_SNAPSHOT_MAJOR=14
 RUN mkdir gcc && cd gcc && \