buildroot: set container env variable

Set the container environment variable to indicate to programs
inside the build root that they are indeed running inside a
container (see also https://systemd.io/CONTAINER_INTERFACE/).

osbuild/buildroot.py

@@ -275,6 +275,7 @@ class BuildRoot(contextlib.AbstractContextManager):
 
         # Setup a new environment for the container.
         env = {
+            "container": "bwrap-osbuild",
             "LC_CTYPE": "C.UTF-8",
             "PATH": "/usr/sbin:/usr/bin",
             "PYTHONPATH": "/run/osbuild/lib",

test/mod/test_buildroot.py

@@ -214,6 +214,7 @@ def test_env_isolation(tempdir):
 
     allowed = [
         "_",      # added by `env` itself
+        "container",
         "LC_CTYPE",
         "PATH",
         "PWD",