Document usage on Container Linux

[jzelinskie]

There's an incompatibility between the assumptions around dynamic linking with pyinstaller and the CoreOS userspace. We should add a small doc recommending that people use the toolbox command in Container Linux to get around this.

Users get an error similar to this:

core@jump ~ $ helm registry login -u $USERNAME quay.io
sh: symbol lookup error: sh: undefined symbol: rl_signal_event_hook
sh: symbol lookup error: sh: undefined symbol: rl_signal_event_hook
sh: symbol lookup error: sh: undefined symbol: rl_signal_event_hook
sh: symbol lookup error: sh: undefined symbol: rl_signal_event_hook

To be clear, there's nothing to fix here, we should just document this.