Fix python installation in expo publisher dockerfile

[imjoehaines]

Goal

There is no "python" package in Alpine linux; it has "python2" and "python3" instead

I'm not sure how this worked previously, but it has started complaining that the "python" package doesn't exist: https://buildkite.com/bugsnag/bugsnag-js-expo/builds/3511#f003482c-cf37-4312-8218-3ce0bad1ab66/191-222

All of our other dockerfiles that install Python use "python3"

[github-actions]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	41.54 kB	12.79 kB
After	41.54 kB	12.79 kB
±	No change	No change

code coverage diff

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0%(+0%)	0%(+0%)	0%(+0%)	0%(+0%)

Generated by 🚫 dangerJS against 272505b

[djskinner]

See https://alpinelinux.org/posts/Alpine-3.12.0-released.html

There is no longer a python package providing python2. You need to explicitly install python2 or python3. Additionally, as part of the ongoing work to remove python2, many python2 modules have been removed.