Add missing Alpine 3.9 image for node.js v13

[PeterDaveHello]

Not sure why I missed it, but I think we sould add it back.

cc #1135

[nschonni]

I don't think it was added because it wasn't an active release when 13 came out. The same reason it doesn't have a Jessie release. Alpine mainly seems to just have a single supported release at a time.
It would probably be good to write down the criteria. I know the Build group has a stricter guideline in that the Debian release needs to be supported for the whole lifetime of the Node release to be supported

[PeterDaveHello]

@nodejs/docker I think we should invite @nschonni to join our team as an active contributor.

[SimenB]

I agree with @nschonni that we shouldn't add this. We should be able to leave old distros behind for new majors of node

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@nodejs/docker I think we should invite @nschonni to join our team as an active contributor.

I totally agree 👍

[nschonni]

I just split off the onboarding discussion for me to a different issue to keep it separate.
I think there is probably some value in syncing or pointing between the build repo about the supported releases. Alpine is a little bit of a wildcard for LTS type releases.

[LaurentGoderre]

Yeah, I don't think it's missing. It wouldn't be much impact on us but why not wait to see if anyone requests it first?

[GrahamCampbell]

I don't think we really need this. Similarly, when node 14 is released, there won't be a 3.9 image.

[GrahamCampbell]

The same situation is with the other official docker images. The Python 3.8 and 3.9 images are only available in the newer dists.

[Ugzuzg]

The main thing that is stopping people from using latest Alpine is this issue: #1158
I think, it's reasonable to support 3.9 until that issue is fixed.

[LaurentGoderre]

@Ugzuzg if using Alpine 3.9 really does fix this issue then that is a use case for bringing it in. Do you have some evidence that it would?

[Ugzuzg]

@LaurentGoderre, we are not seeing this error anymore.