Use the machine executor in CircleCI for Linux

[Vkt0r]

• Use the machine executor (https://circleci.com/docs/2.0/executor-types/#using-machine) which gives us 8GB of RAM and avoid the issues with the Linux Docker image.
• Add a Dockerfile and the .dockerignore with all the details to run Docker with Swift 5.0 in Linux

[swifter-bot]

	2 Warnings
⚠️	It seems like you’ve added new tests to the library. If that’s the case, please update the XCTestManifests.swift file running in your terminal the command swift test --generate-linuxmain.
⚠️	If you ran the command swift test --generate-linuxmain in your terminal, please remove the line testCase(IOSafetyTests.__allTests__IOSafetyTests), from public func __allTests() -> [XCTestCaseEntry] in the bottom of the file. For more reference see #366.

	1 Message
📖	Hey, @Vkt0r 👋.

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