CI: Show execution environment before we start

This will help to debug CI issues that are related to us running in
a container that might have unusual capabilities, mount points,
filesystems etc., such as (probably) #2027, #2028, #2029.

Signed-off-by: Simon McVittie <smcv@collabora.com>

.gitlab-ci/show-execution-environment.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+set -eux -o pipefail
+
+id || :
+capsh --print || :
+env -0 | sort -z | perl -pe 's/\0/\n/g' || :
+setpriv --dump || :
+ulimit -a || :
+cat /proc/self/status || :
+cat /proc/self/mountinfo || :