_image_needs_[building|pushing]: check platforms when using docker buildx

[manics]

Closes #132

Not yet tested....

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Update by Erik

Tested and it works great!

[consideRatio]

Since we want to work with a frozen set rather than a l list, should we perhaps instead convert the platforms list to a frozen set directly after args = argparser.parse_args(args) and adjust various functions relating to platform?

Or hmm, i see build_images expect a list based on the docs, and is the only recipient of platforms, so perhaps to convert it to a frozenset within build_image.

-    if not platforms:
-        platforms = []
+    if platforms:
+        platforms = frozenset(platforms)
+    else:
+        platforms = frozenset()

chartpress/chartpress.py

Lines 339 to 340 in d625f46

	if not platforms:
	platforms = []

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From some code inspection, this seems viable to do. Is that a sensible change? it would enable us to have a single function instead of two separate, and still wouldn't be a breaking API change for build_images.

[consideRatio]

@manics this looks good, and it successfully opted to not push when i tried out the reproduction steps in #132 (comment) (that i had small errors in, but fixed them just now).

If you think its worth the time and effort, and it is a sensible idea to refactor platforms to be a frozenset before the helper functions checking if build/push needs to be done are called, then do that but otherwise I think we should go for a merge!

[manics]

Good point, done!

[consideRatio]

@manics wieeee!!! Thanks for your work on multi platform stuff, I love it!!