Add .nvmrc to ensure active LTS is used in Netlify builds

[chalin]

Otherwise, the default version of node that is currently used is v8. For example, see this build log: https://app.netlify.com/sites/containerd-io/deploys/62dae776ae6e5900087c5111

Now using node v8.17.0

For details, see https://docs.netlify.com/configure-builds/manage-dependencies/#node-js-and-javascript.

[chalin]

/assign @chalin

In the meantime, I've set the NODE_VERSION to lts/* in the Netlify site config.