"mysql_config: not found" but Dockerfile does include "libmysqlclient-dev"

[FrancescoCasalegno]

Details

• Read the Docs project URL: https://readthedocs.org/projects/blue-brain-search/
• Build URL (if applicable): https://readthedocs.org/projects/blue-brain-search/builds/13169816/
• Read the Docs username (if applicable): FrancescoCasalegno

Expected Result

I would expect pip install .[dev] to run successfully without any issue.

The only tricky point is the mysqlclient dependency in the setup.py, because this package requires a C system library and I read that RTD "does not support installing random C libraries".
However this should not be an issue for the following reasons:

1. The only C system library I need is libmysqlclient-dev and the RTD Dockerfile seems to include it
2. I tried to docker pull readthedocs/build:latest and run the various commands locally, and everything worked fine. The mysql_config is also clearly there:

docs@27e903412d0f:/$ file /usr/bin/mysql_config 
/usr/bin/mysql_config: POSIX shell script, ASCII text executable

Actual Result

The RTD build fails when trying to install the mysqlclient dependency, saying mysql_config: not found.

[humitos]

I'm closing this issue we can continue the discussion in readthedocs/readthedocs-docker-images#158