Dockerfile detector will only check files containing "Dockerfile" in the name

[spectrox]

Fixes #3484.

Description

Initializer tries to find all available builders by checking each file in the tree for compatible with Dockerfile syntax.
This syntax is not unique and python file can be taken for Dockerfile:

from django.test import TestCase

This fix adds additional check to the file name which should contain "Dockerfile".

I am not sure if that is appropriate solution, because it can affect existing users.
But as far as I know, developers tend to keep Dockerfile in the name even when using multiple files in the same folder.

User facing changes

Before

When running skaffold init with python files like the one mentioned above you can see them in the list of available builders.

After

No python files should be listed in the survey.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Mentions any output changes.
• Adds documentation as needed: user docs, YAML reference, CLI reference.
• Adds integration tests if needed.

Reviewer Notes

• The code flow looks good.
• Unit test added.
• User facing changes look good.

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[spectrox]

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[codecov]

Codecov Report

Merging #3499 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
pkg/skaffold/initializer/init.go	63.63% <100%> (+0.22%)	⬆️
pkg/skaffold/util/tar.go	52.87% <0%> (-4.6%)	⬇️
...affold/kubernetes/portforward/kubectl_forwarder.go	68.29% <0%> (+2.43%)	⬆️

[dgageot]

FWIW, I've also seen a lot of *.dockerfile out there

[spectrox]

so, it would be better to make it case insensitive?

[spectrox]

I've made it case insensitive and updated test to cover this case.

[cedrickring]

We could also check in docker.Validate(path) if there is some python-ish code after from ... (like import <name>).

WDYT @dgageot

[spectrox]

I don't think that Dockerfile validator should be aware of python-ish code constructs.
It's responsibility to validate Dockerfile and it does that by validating commands. Validating arguments would be too much overhead IMO.