Moved the environment variables to the top of the Dockerfile

[LaurentGoderre]

No description provided.

[chorrell]

LGTM

[yosifkit]

What was the reason for this change? This previously allowed multiple versions of node to share the gpg fetching layer if they had the same keys and base image, now that is not possible. 😢

[chorrell]

It was just cosmetic. I don't think any of us realized the proximity of the ENV vars to RUN would matter...

@LaurentGoderre I think we should revert this. I think this might be the cause of the sporadic gpg fetching errors I saw in the test runs

I'm going to do a follow up PR to switch this back, I want to get that landed before v6.0.0

[chorrell]

This was reverted in #167

[yosifkit]

Thanks, docker build caching and layer sharing between versions in great to have. 😄 And hopefully this makes your tests run smoother. 🎉

Technically you can move other docker metadata commands like EXPOSE to the top of the file but we usually put them later for logical Dockerfile flow and better readability, since those type of layers are tiny anyway.