Fix Dockerfile parsing on Windows

Line endings are written as "\r" by default on Windows, which makes the
Dockerfiles unreadable by Docker.

See: https://github.com/overhangio/tutor/pull/385#issuecomment-730387969

CHANGELOG.md

@@ -4,6 +4,8 @@ Note: Breaking changes between versions are indicated by "💥".
 
 ## Unreleased
 
+- [Bugfix] Fix Dockerfile parsing on Windows
+
 ## v10.5.0 (2020-11-19)
 
 - 💥[Improvement] Remove `dev/local pullimages`. Instead, run `dev/local dc pull`.

tutor/commands/plugins.py

@@ -117,7 +117,7 @@ def install(location):
     # Save file
     if not os.path.exists(plugins.DictPlugin.ROOT):
         os.makedirs(plugins.DictPlugin.ROOT)
-    with open(plugin_path, "w") as f:
+    with open(plugin_path, "w", newline="\n") as f:
         f.write(content)
     fmt.echo_info("Plugin installed at {}".format(plugin_path))
 

tutor/env.py

@@ -227,13 +227,15 @@ def write_to(content, path):
     """
     Write some content to a path. Content can be either str or bytes.
     """
-    open_mode = "w"
-    encoding = "utf8"
+    open_kwargs = {"mode": "w"}
     if isinstance(content, bytes):
-        open_mode += "b"
-        encoding = None
+        open_kwargs["mode"] += "b"
+    else:
+        # Make files readable by Docker on Windows
+        open_kwargs["encoding"] = "utf8"
+        open_kwargs["newline"] = "\n"
     utils.ensure_file_directory_exists(path)
-    with open(path, open_mode, encoding=encoding) as of:
+    with open(path, **open_kwargs) as of:
         of.write(content)