fresh install with all-in-one docker container following the doc "Using this container in production"
https://www.openproject.org/docs/installation-and-operations/installation/docker/

creating the container just as told :

sudo mkdir -p /var/lib/openproject/{pgdata,assets} 

docker run -d -p 8080:80 --name openproject \
  -e OPENPROJECT_HOST__NAME=mywebsite.com \
  -e OPENPROJECT_SECRET_KEY_BASE=dsfidfoehzoifeozifjoze \
  -v /var/lib/openproject/pgdata:/var/openproject/pgdata \
  -v /var/lib/openproject/assets:/var/openproject/assets \
  openproject/community:12

verifying it's running good with 
docker ps
docker top openproject

everything runs nicely

using apache2 with openproject.conf just as the docs says (replacing ServerName with mine); and using correct .pem and .key from CloudFlare :

<VirtualHost *:80>
    ServerName mywebsite.com

    RewriteEngine on
    RewriteCond %{HTTPS} !=on
    RewriteRule ^/?(.*)$ https://%{SERVER_NAME}/$1 [R,L]
</VirtualHost>

<VirtualHost *:443>
    ServerName mywebsite.com

    SSLEngine on
    SSLCertificateFile /etc/ssl/certs/server.pem
    SSLCertificateKeyFile /etc/ssl/private/server.key

    RewriteEngine on
    RewriteRule "^$" "/" [R,L]

    ProxyRequests off

    <Location "/">
      RequestHeader set X-Forwarded-Proto 'https'

      ProxyPreserveHost On
      ProxyPass http://127.0.0.1:8080/
      ProxyPassReverse http://127.0.0.1:8080/
    </Location>
</VirtualHost>

Finally, when typing mywebsite.com,  I get :
" ERR_TOO_MANY_REDIRECTS "

When typing https://01.01.01.01 (my server ip) I get :
Apache default page

When typing http://01.01.01.01 (my server ip without ssl ) I get :
another of my websites (the f1rst alphabetically)

3 days on it, is the doc written correctly ?