use official docker image

[tomerd]

motivation: now that swift has official docker images, use them instead of the homebrewed one

changes: refactor docker file to use swift:version instead of ubuntu:version

[tomerd]

i plan to do the same for swift-nio once we have docker files for nightlies

[Lukasa]

Seems to have a build problem.

[tomerd]

@swift-nio-bot test this please

[tomerd]

@Lukasa seems like some of the tests are failing on performance

00:05:16.251 Test Case 'HuffmanCodingTests.testBasicDecodingPerformance' started at 2019-05-21 23:15:55.744
00:05:23.502 /code/Tests/NIOHPACKTests/HuffmanCodingTests.swift:147: Test Case 'HuffmanCodingTests.testBasicDecodingPerformance' measured [Time, seconds] average: 0.643, relative standard deviation: 33.854%, values: [0.601727, 0.458943, 0.521925, 0.647700, 0.465004, 0.466388, 0.578569, 0.896835, 1.137924, 0.658226], performanceMetricID:org.swift.XCTPerformanceMetric_WallClockTime, maxPercentRelativeStandardDeviation: 10.000%, maxStandardDeviation: 0.100
00:05:23.502 /code/Tests/NIOHPACKTests/HuffmanCodingTests.swift:147: error: HuffmanCodingTests.testBasicDecodingPerformance : failed: The relative standard deviation of the measurements is 33.854% which is higher than the max allowed of 10.000%.
00:05:23.502 Test Case 'HuffmanCodingTests.testBasicDecodingPerformance' failed (7.251 seconds)
00:05:23.502 Test Case 'HuffmanCodingTests.testComplexEncodingPerformance' started at 2019-05-21 23:16:02.996
00:05:27.555 /code/Tests/NIOHPACKTests/HuffmanCodingTests.swift:164: Test Case 'HuffmanCodingTests.testComplexEncodingPerformance' measured [Time, seconds] average: 0.367, relative standard deviation: 15.216%, values: [0.400150, 0.316402, 0.286368, 0.383660, 0.390134, 0.284481, 0.392024, 0.458026, 0.402609, 0.356516], performanceMetricID:org.swift.XCTPerformanceMetric_WallClockTime, maxPercentRelativeStandardDeviation: 10.000%, maxStandardDeviation: 0.100
00:05:27.555 Test Case 'HuffmanCodingTests.testComplexEncodingPerformance' passed (4.052 seconds)
00:05:27.555 Test Case 'HuffmanCodingTests.testComplexDecodingPerformance' started at 2019-05-21 23:16:07.047
00:05:39.386 /code/Tests/NIOHPACKTests/HuffmanCodingTests.swift:194: Test Case 'HuffmanCodingTests.testComplexDecodingPerformance' measured [Time, seconds] average: 1.064, relative standard deviation: 12.453%, values: [1.426129, 1.005346, 1.002405, 1.005888, 1.012476, 1.006756, 1.003084, 1.120459, 1.042937, 1.010492], performanceMetricID:org.swift.XCTPerformanceMetric_WallClockTime, maxPercentRelativeStandardDeviation: 10.000%, maxStandardDeviation: 0.100
00:05:39.386 /code/Tests/NIOHPACKTests/HuffmanCodingTests.swift:194: error: HuffmanCodingTests.testComplexDecodingPerformance : failed: The relative standard deviation of the measurements is 12.453% which is higher than the max allowed of 10.000%.
00:05:39.386 Test Case 'HuffmanCodingTests.testComplexDecodingPerformance' failed (11.832 seconds)
00:05:39.386 Test Suite 'HuffmanCodingTests' failed at 2019-05-21 23:16:18.879
00:05:39.386 	 Executed 6 tests, with 2 failures (0 unexpected) in 26.947 (26.947) seconds

@swift-nio-bot test this please