Thanks a lot for the report. Do you have a complete backtrace? (you can get one by passing --traceback to Alectryon). The reason I'm asking is that Alectryon doesn't really print much to stdout, so this error seems to mean that programs in that docker container can't even write files that contain non-ascii characters.

I think the solution is here: https://stackoverflow.com/questions/52065842/python-docker-ascii-codec-cant-encode-character (ignore the incorrect duplicate banner)

I wonder if this is the same problem as the one that forced @jfehrle to catch encoding exceptions in https://github.com/coq/coq/pull/13564/files#diff-99858e5d76716d34bcaf9ad38b8d67f05a7a8849e7969faa8b2318805d94f223R219 .

Also, I believe this means the build will break anytime anyone uses an UTF8 character in a Coq file. […]
I theorize that one more complete workaround would be to set up a locale (e.g., en_US.UTF8) in the Docker container, but this seems like a cumbersome thing to do

I think that's the right solution, precisely because of your point on non-ascii characters in Coq files. Fortunately it looks easy (ENV LANG en_US.utf8); once we confirm that this works, I'll add a note in the readme.

Complete command and backtrace from inside the container:

user@eaac613822d7:~/casper-cbc-proofs$ ~/alectryon/alectryon.py --frontend coqdoc --webpage-style windowed --traceback -Q . CasperCBC --output-directory tmp Lib/Classes.v
Traceback (most recent call last):
  File "/home/user/alectryon/alectryon.py", line 26, in <module>
    main()
  File "/home/user/alectryon/alectryon/cli.py", line 631, in main
    process_pipelines(args)
  File "/home/user/alectryon/alectryon/cli.py", line 623, in process_pipelines
    raise e
  File "/home/user/alectryon/alectryon/cli.py", line 620, in process_pipelines
    state = call_pipeline_step(step, state, ctx)
  File "/home/user/alectryon/alectryon/cli.py", line 589, in call_pipeline_step
    return step(state, **{p: ctx[p] for p in params})
  File "/home/user/alectryon/alectryon/cli.py", line 326, in <lambda>
    write_output(ext, contents, fname, output, output_directory)
  File "/home/user/alectryon/alectryon/cli.py", line 322, in write_output
    f.write(contents)
UnicodeEncodeError: 'ascii' codec can't encode character '\u2191' in position 6441: ordinal not in range(128)

@cpitclaudel it actually seems as though the following diff for cli.py solves the issue completely, even with LANG=C:

@@ -318,7 +318,7 @@ def write_output(ext, contents, fname, output, output_directory):
     else:
         if not output:
             output = os.path.join(output_directory, strip_extension(fname) + ext)
-        with open(output, mode="w") as f:
+        with open(output, mode="w", encoding="utf-8") as f:
             f.write(contents)
 
 def write_file(ext):

Since the whole project is supposed to be UTF8 anyway, would a PR with this change be welcome? To me, this would be a better fix than remembering to change LANG everywhere.