PHP 8 support

Also updates AWS SDK dependency version and dockerfile to point to the new, correct port for test runs

Author: Will Jones

composer.json

@@ -3,7 +3,7 @@
     "description": "A PHP library that can be used to interact with Amazon DynamoDB",
     "license": "MIT",
     "require": {
-        "php": "^7.2",
+        "php": "^7.2||~8.0.0",
         "aws/aws-sdk-php": "^3.95",
         "guillermoandrae/php-collection": "^1.0"
     },