Handle requirements.txt with `--pre` lines

This is a global flag which our inspection of requirements.txt files
should ignore as it doesn't imply a local reference.

repo2docker/utils.py

@@ -469,6 +469,10 @@ def is_local_pip_requirement(line):
     if line.startswith(("--requirement", "--constraint")):
         # as above but flags are spelt out
         return True
+    # the `--pre` flag is a global flag and should appear on a line by itself
+    # we just care that this isn't a "local pip requirement"
+    if line.startswith("--pre"):
+        return False
     # strip off things like `--editable=`. Long form arguments require a =
     if line.startswith("--"):
         line = line.split("=", 1)[1]

tests/unit/test_utils.py

@@ -146,6 +146,8 @@ def test_open_guess_encoding():
         ("git+https://github.com/jupyterhub/repo2docker", False),
         ("numpy", False),
         ("# -e .", False),
+        ("--pre", False),
+        ("--pre pandas", False),
     ],
 )
 def test_local_pip_requirement(req, is_local):