`Dockerfile` based repositories fail to start up, citing "Error unpacking user from cookie"

[wallyhall]

Bug description

Building from a repo with a Dockerfile (so this may be a repo2docker issue, but I don't understand it enough to know where to raise this other than here) fails to start due to "invalid/expired cookie" when "handing over" to JupyterLab/Notebook in browser.

Example on mybinder.org: https://mybinder.org/v2/gh/binder-examples/minimal-dockerfile/HEAD

I've reproduced the issue on 1.0.0-0.dev.git.2937.h0f65f33 locally too, using our own Dockerfile.

Logs from the "spun up" container created by repo2docker (and orchestrated by Binder):

[W 16:12:41.062 NotebookApp] Not allowing login redirect to '/user/evonetix-xxx-nt805vjb/lab/?'
[I 16:12:41.065 NotebookApp] 302 POST /user/evonetix-xxx-nt805vjb/login?next=%2Fuser%2Fevonetix-xxx-nt805vjb%2Flab%2F%3F (::ffff:10.100.100.120) 4.270000ms
[I 16:12:41.088 NotebookApp] 302 GET /user/evonetix-xxx-nt805vjb/ (::ffff:****) 0.830000ms
[E 16:12:41.111 NotebookApp] Error unpacking user from cookie: Expecting value: line 1 column 1 (char 0)
[W 16:12:41.112 NotebookApp] Clearing invalid/expired login cookie username-hub-binder-evonetix-net
[I 16:12:41.115 NotebookApp] 302 GET /user/evonetix-xxx-nt805vjb/lab/? (::ffff:****) 4.620000ms

On mybinder.org you get a pretty error message. On ours locally, we get the Jupyter "login" screen, asking for a token.

Providing the token (as extracted from kubectl describe pod ...) just bumps back to the same screen with no error. The logs contain the above.

Expected behaviour

We expect the Jupyter notebook to appear!

Actual behaviour

We get a Jupyter login page asking for the token again (with no error) ... or an error page on mybinder.org.

How to reproduce

Try to start a repo with a Dockerfile, for example: https://mybinder.org/v2/gh/binder-examples/minimal-dockerfile/HEAD

We've tried our own Dockerfile too, carefully following the Binder documentation on the subject.

Have tried specifying JupyterLab 3.4.8 in via our requirements.txt explicitly too, in case it was an incompatibility with newer versions. Same problem persists.