cli: show error when popper run -e singularity runs in docker (#899)

throw error message to notify users when attempting to use the singularity runner
when popper is being executed inside a docker container.

In order to identify this, the SingularityRunner constructor looks at /proc/1/cgroup
to check if docker folders are available, which is an indication that a process is 
being executed inside a docker container (https://stackoverflow.com/a/20012536)

The message instructs users to install popper via PIP instead.

src/popper/runner_host.py

@@ -312,6 +312,19 @@ def __init__(self, init_spython_client=True, **kw):
         self._spawned_containers = set()
         self._s = None
 
+        if SingularityRunner._in_docker():
+            log.fail(
+                (
+                    "You seem to be running Popper in a Docker container.\n"
+                    "Singularity cannot be executed this way.\n"
+                    "Either run Popper without Singularity or install Popper "
+                    "through PIP.\n"
+                    "Instructions are available here:\n"
+                    "https://github.com/getpopper/popper/"
+                    "blob/master/docs/installation.md"
+                )
+            )
+
         if self._config.reuse:
             log.fail("Reuse not supported for SingularityRunner.")
 
@@ -361,6 +374,13 @@ def _get_recipe_file(build_ctx_path, cid):
         else:
             log.fail("No Dockerfile was found.")
 
+    @staticmethod
+    def _in_docker():
+        """ Returns TRUE if we are being executed in a Docker container. """
+        if os.path.isfile("/proc/1/cgroup"):
+            with open("/proc/1/cgroup", "r") as f:
+                return "docker" in f.read() or "lxc" in f.read()
+
     def _build_from_recipe(self, build_ctx_path, build_dest, cid):
         SingularityRunner.lock.acquire()
         pwd = os.getcwd()