Add unofficial build notice for Node Alpine

[LaurentGoderre]

[tianon]

I'm guessing you excluded the latter two paragraphs of https://github.com/docker-library/docs/blob/dcac2322f4db6bbf2cfdc2fc726298aae1355443/.template-helpers/variant-alpine.md intentionally -- I'm not opposed, but maybe you can elaborate on why you didn't include them? Aren't they still relevant to Node's Alpine variants?

[SimenB]

Should we link to https://github.com/nodejs/node/blob/main/BUILDING.md#platform-list somewhere, to let people know musl support is considered experimental by the node project?

[tianon]

@Starefossen @hmalphettes @jlmitch5 / @mikeal @SimenB @PeterDaveHello (https://github.com/nodejs/docker-node#governance-and-current-members)

Any objection to merging this?

[PeterDaveHello]

Just a very small question, but overall, I'm good with it.