Fix #117: Remove Conan from Dev Container (unused?) (#152)

* Fix #117: remove unused Conan install from dev container

Conan was installed in `.devcontainer/Dockerfile` but never used; the
project manages its dependencies via CPM (see Dependencies.cmake), so
the `pip install conan` step and the `CONAN_SYSREQUIRES_*` environment
variables were dead weight on every dev container build. Drop them, and
add a CTest regression test that scans the Dockerfile to prevent Conan
from being reintroduced.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Address review: remove devcontainer Conan-removal regression test

Requested by @lefticus in PR #152 review.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: leftibot <leftibot@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: leftibot

.devcontainer/Dockerfile

@@ -13,20 +13,6 @@ RUN apt-get update -qq && export DEBIAN_FRONTEND=noninteractive && \
         make ninja-build git \
         python3 python3-pip
 
-# Install conan
-RUN python3 -m pip install --upgrade pip setuptools && \
-    python3 -m pip install conan && \
-    conan --version
-
-# By default, anything you run in Docker is done as superuser.
-# Conan runs some install commands as superuser, and will prepend `sudo` to
-# these commands, unless `CONAN_SYSREQUIRES_SUDO=0` is in your env variables.
-ENV CONAN_SYSREQUIRES_SUDO 0
-# Some packages request that Conan use the system package manager to install
-# a few dependencies. This flag allows Conan to proceed with these installations;
-# leaving this flag undefined can cause some installation failures.
-ENV CONAN_SYSREQUIRES_MODE enabled
-
 # User-settable versions:
 # This Dockerfile should support gcc-[7, 8, 9, 10, 11] and clang-[10, 11, 12, 13]
 # Earlier versions of clang will require significant modifications to the IWYU section