fix(docker): don't open files in stream_context_tar

Opening file paths here results in an OSError in Gecko due to the large
number of files involved in creating docker images there.

The underlying tar writer accepts both file paths or file objects, so
switch to paths to avoid the error in Gecko.

Author: ahal

src/taskgraph/util/docker.py

@@ -149,7 +149,7 @@ def stream_context_tar(topsrcdir, context_dir, out_file, image_name=None, args=N
         for f in files:
             source_path = os.path.join(root, f)
             archive_path = source_path[len(context_dir) + 1 :]
-            archive_files[archive_path] = open(source_path, "rb")
+            archive_files[archive_path] = source_path
 
     # Parse Dockerfile for special syntax of extra files to include.
     content = []