[ubuntu] feat(docker): authenticate using DockerHub credentials (#2548)

Ubuntu builds, by default, pull images anonymously from the
official DockerHub. DockerHub rate limits requests from
anonymous and unpaid accounts:
- https://docs.docker.com/docker-hub/download-rate-limit/

When those rate limits are reached the Packer build will
fail with an error indicated the rate limit has been
reached.

Add support for providing credentials for a paid
account that can be used by the Docker setup steps to
authenticate when pulling images from DockerHub, thereby
avoiding the rate limit.

Related to #2094

images/linux/ubuntu1604.json

@@ -20,7 +20,9 @@
         "capture_name_prefix": "packer",
         "image_version": "dev",
         "image_os": "ubuntu16",
-        "run_validation_diskspace": "false"
+        "run_validation_diskspace": "false",
+        "dockerhub_login": "{{env `DOCKERHUB_LOGIN`}}",
+        "dockerhub_password": "{{env `DOCKERHUB_PASSWORD`}}"
     },
     "sensitive-variables": [
         "client_secret"
@@ -166,6 +168,20 @@
             ],
             "execute_command": "sudo sh -c '{{ .Vars }} pwsh -f {{ .Path }}'"
         },
+        {
+            "type": "shell",
+            "scripts": [
+                "{{template_dir}}/scripts/installers/docker-compose.sh",
+                "{{template_dir}}/scripts/installers/docker-moby.sh"
+            ],
+            "environment_vars": [
+                "HELPER_SCRIPTS={{user `helper_script_folder`}}",
+                "INSTALLER_SCRIPT_FOLDER={{user `installer_script_folder`}}",
+                "DOCKERHUB_LOGIN={{user `dockerhub_login`}}",
+                "DOCKERHUB_PASSWORD={{user `dockerhub_password`}}"
+            ],
+            "execute_command": "sudo sh -c '{{ .Vars }} {{ .Path }}'"
+        },
         {
             "type": "shell",
             "scripts": [
@@ -181,8 +197,6 @@
                 "{{template_dir}}/scripts/installers/swift.sh",
                 "{{template_dir}}/scripts/installers/cmake.sh",
                 "{{template_dir}}/scripts/installers/codeql-bundle.sh",
-                "{{template_dir}}/scripts/installers/docker-compose.sh",
-                "{{template_dir}}/scripts/installers/docker-moby.sh",
                 "{{template_dir}}/scripts/installers/dotnetcore-sdk.sh",
                 "{{template_dir}}/scripts/installers/erlang.sh",
                 "{{template_dir}}/scripts/installers/firefox.sh",
@@ -351,4 +365,4 @@
             "execute_command": "sudo sh -c '{{ .Vars }} {{ .Path }}'"
         }
     ]
-}
+}