Merge branch 'linux_fds'

Author: Brandon Philips

runtime-linux.md

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+## File descriptors
+By default, only the `stdin`, `stdout` and `stderr` file descriptors are kept open for the application by the runtime.
+
+The runtime may pass additional file descriptors to the application to support features such as [socket activation](http://0pointer.de/blog/projects/socket-activated-containers.html).
+
+Some of the file descriptors may be redirected to `/dev/null` even though they are open.