Converted starter script for docker tests to cross-platform powershell (#877)

Author: Bart Koelman

prop-docker.sh

@@ -0,0 +1,12 @@
+#Requires -Version 7.0
+
+# This script starts a docker container with postgres database, used for running tests.
+
+docker container stop jsonapi-dotnet-core-testing
+
+docker run --rm --name jsonapi-dotnet-core-testing       `
+ -e POSTGRES_DB=JsonApiDotNetCoreExample                 `
+ -e POSTGRES_USER=postgres                               `
+ -e POSTGRES_PASSWORD=postgres                           `
+ -p 5432:5432                                            `
+ postgres:12.0