Add support for apko publish --local

[jdolitsky]

Resolves #419

Running something like

apko publish --local x.yaml r.example.com/myimage:a r.example.com/myimage:b

Results in

$ docker images | grep -E 'apko.local|example'
apko.local/cache                     50309e72bb62d5a3fbf446da4b3b5044a6eb147567bdcd00c8b1e2430629b587   aec73839e4e3   52 years ago   7.16MB
apko.local/cache                     d6df0a217a044c27f6876099edcab51dbfdd932d46e9277a45d95be570148d47   bc88f613cebb   52 years ago   7.04MB
r.example.com/myimage                a                                                                  bc88f613cebb   52 years ago   7.04MB
r.example.com/myimage                b                                                                  bc88f613cebb   52 years ago   7.04MB
apko.local/cache                     62f96af646358ed0b57739d7c531b38885e4793dceec0445e2de00243d8ab72f   543277880ce1   52 years ago   8.43MB
apko.local/cache                     35f0248e7688bf583fd74e1dd7bd650e22770466d8771e20fb1587524360a99a   663ae2bddd26   52 years ago   5.2MB
apko.local/cache                     6d68636fb4396aa64820c18e7a36b137539b885745650291cec99bc62c6e338f   73aec04a23bc   52 years ago   7.44MB
apko.local/cache                     c1154d59ddd8dea59da34dbfeab976eb662fbc9b2caaa2c7424b487fa0ca297f   9f0b9218c63b   52 years ago   6.91MB
apko.local/cache                     4b973e8e01b311259fb928bf2295dd3d1c4530e842fe732e16eb08f09d22f75c   a251d7022b61   52 years ago   5.48MB
apko.local/cache                     e2601f4d431da892e85ae988ef56e76c925b1c5c20c18a0bbf308f44188353a8   06bb582eb120   52 years ago   6.64MB

[kaniini]

NAILED IT