Remove node6

[koooge]

Hello ✋ . redash/base doesn't have to install node. refs getredash/redash#3199

[arikfr]

Thanks!

We need to think how to handle this: currently all the old getredash/redash Dockerfiles reference redash/base:latest, which if we merge this will no longer have Node installed.

I think we should:

1. Switch to versioned image references.
2. Keep latest tag reference the old image until enough time passed and we can deprecate it.

[arikfr]

Until I figure out how to build versions, let's keep this open.