Fix Kubernetes container detection

[lbogdan]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information:

While configuring a new Kubernetes cluster for CodeSandbox Containers, I've noticed that HMR was not working (again, see #2795). The reason seems to be that cgroup naming has changed in recent Kubernetes versions, and the regex at

vue-cli/packages/@vue/cli-service/lib/commands/serve.js

Line 300 in 11d60d5

return /:\/(lxc|docker|kubepods)\//.test(content)

doesn't match anymore. This PR fixes the regex to match the new cgroup name, adding an optional .slice after kubepods.

I also went ahead and added a special detection for CodeSandbox Containers, by checking if the CODESANDBOX_SSE environment variable is set, so that we can control detection regardless of future cgroup naming changes (or even containerization technology). We would appreciate getting this additional check in, however we can understand if you're hesitant to add 3rd party logic, so just let me know and I'll remove it.

Although it's hard to test the checkInContainer() function, you can see this PR working in our new cluster at https://4iycu.sse.codesandbox.stream/ (with serve.js manually patched inside node_modules).

[haoqunjiang]

Codesandbox is a common execution environment for Vue CLI so I'm fine with the environment variable.

But it would be nice if you can document that variable somewhere in the codesandbox documentation so that future contributors can better understand the logic here.

[lbogdan]

Thanks, we'll update the documentation today!