Use the machine executor

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
* Update the CHANGELOG

.circleci/config.yml

@@ -61,10 +61,15 @@ jobs:
           path: /tmp/test-results
 
   Test Linux Platform:
-    docker:
-      - image: swift:5.0
+    machine: true
     steps:
       - checkout
+      - run:
+          name: Build Docker Image
+          command: docker build --tag swift5.0 .
+      - run:
+          name: Run Docker Container
+          command: docker run --rm swift5.0
       - run: 
           name: Run Unit Tests
           command: swift test

.dockerignore

@@ -0,0 +1,5 @@
+.git/
+.build/
+!.build/checkouts/
+!.build/repositories/
+!.build/workspace-state.json

CHANGELOG.md

@@ -22,6 +22,10 @@ All notable changes to this project will be documented in this file. Changes not
 
 - Add the `trailing_whitespace` rule in Swiftlint and autocorrect all the source files. ([#416](https://github.com/httpswift/swifter/pull/416)) by [@Vkt0r](https://github.com/Vkt0r)
 
+## Changed
+
+- 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. ([#422](https://github.com/httpswift/swifter/pull/422)) by [@Vkt0r](https://github.com/Vkt0r)
+
 # [1.4.7] 
 
 ## Added

Dockerfile

@@ -0,0 +1,9 @@
+FROM swift:5.0
+
+WORKDIR /package
+
+COPY . ./
+
+RUN swift package resolve
+RUN swift package clean
+CMD swift test