Enable Chromium to run on devcontainer

The Chromium browser has to run in headless mode when we are running in
a devcontainer (e.g. GitHub Codespaces).

Also, we need to run the container without [seccomp][1] restrictions,
otherwise we receive an error when running the specifications, saying
that there is no usable sandbox for Chromium to run in.

It would be nice if we could use a [custom seccomp profile][2], rather
than removing all seccomp restrictions entirely, but it appears that
Codespaces doesn't currently allow custom seccomp profiles. Certainly
the [docs for the (now-obsolete) Visual Studio Codespaces say that][3].

[1]: https://docs.docker.com/engine/security/seccomp/
[2]: jlund/docker-chrome-pulseaudio#8 (comment)
[3]: https://docs.microsoft.com/en-us/visualstudio/codespaces/reference/configuring#docker-properties

.devcontainer/devcontainer.json

@@ -6,6 +6,8 @@
 		"args": { "VARIANT": "14" }
 	},
 
+	"runArgs": ["--security-opt", "seccomp=unconfined" ],
+
 	// Set *default* container specific settings.json values on container create.
 	"settings": { 
 		"terminal.integrated.shell.linux": "/bin/bash"

env/default/headless.properties

@@ -1,4 +1,4 @@
 # headless.properties
 
 # Change this to true to run test in headless mode in chrome
-headless_chrome = false
+headless_chrome = true