For those who want to run Onlyoffice in Seafile under Apache web server on Ubuntu 16.04, I share the correct and complet How To.

I suppose you already have a seafile server running by following server manual.

• Install docker in Ubuntu 16.04 amd64 :

$ sudo apt-get update

$ sudo apt-get install \
    apt-transport-https \
    ca-certificates \
    curl \
    software-properties-common

$ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -

$ sudo add-apt-repository \
   "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
   $(lsb_release -cs) \
   stable"

$ sudo apt-get update

$ sudo apt-get install docker-ce

• Deploy OnlyOffice DocumentServer Docker image :

docker run -dit -p 88:80 --restart always --name oods onlyoffice/documentserver

• Configure Apache :

LoadModule authn_core_module modules/mod_authn_core.so
LoadModule authz_core_module modules/mod_authz_core.so
LoadModule proxy_module modules/mod_proxy.so
LoadModule proxy_http_module modules/mod_proxy_http.so
LoadModule proxy_wstunnel_module modules/mod_proxy_wstunnel.so
LoadModule headers_module modules/mod_headers.so
LoadModule setenvif_module modules/mod_setenvif.so
LoadModule ssl_module modules/mod_ssl.so

<IfModule unixd_module>
  User seafile           // replace with your seafile user
  Group seafile          // replace with your seafile group
</IfModule>

<VirtualHost *:80>
   ServerName server.fr  // replace "server.fr" with your server URL
   ServerAlias seafile
   Redirect permanent / https://server.fr/
</VirtualHost>

<VirtualHost *:443>
  ServerName server.fr
  DocumentRoot /var/www

  SSLEngine On
  Include /etc/letsencrypt/options-ssl-apache.conf
  SSLCertificateFile /etc/letsencrypt/live/server.fr/fullchain.pem
  SSLCertificateKeyFile /etc/letsencrypt/live/server.fr/privkey.pem

  Alias /media /haiwen/seafile-server-latest/seahub/media

  <Location /media>
    Require all granted
  </Location>

  RewriteEngine On
  ProxyPass /seafhttp http://127.0.0.1:8082
  ProxyPassReverse /seafhttp http://127.0.0.1:8082

  RewriteRule ^/seafhttp - [QSA,L]
  SetEnvIf Request_URI . proxy-fcgi-pathinfo=unescape
  SetEnvIf Authorization "(.*)" HTTP_AUTHORIZATION=$1

  ProxyPass / http://127.0.0.1:8000/
  ProxyPassReverse / http://127.0.0.1:8000/
  ProxyPass / fcgi://127.0.0.1:8000/
  

Define VPATH /onlyofficeds
Define DS_ADDRESS server.fr:88

<Location ${VPATH}>
Require all granted
  SetEnvIf Host "^(.*)$" THE_HOST=$1
  RequestHeader setifempty X-Forwarded-Proto https   // https VERY important !
  RequestHeader setifempty X-Forwarded-Host %{THE_HOST}e
  RequestHeader edit X-Forwarded-Host (.*) $1${VPATH}
  ProxyAddHeaders Off
  ProxyPass "http://${DS_ADDRESS}/"
  ProxyPassReverse "http://${DS_ADDRESS}/"
</Location>

</VirtualHost>

• Configure Seafile Server for SUBFOLDER :
  Add the following config option to seahub_settings.py:

# Enable Only Office
ENABLE_ONLYOFFICE = True
VERIFY_ONLYOFFICE_CERTIFICATE = True
ONLYOFFICE_APIJS_URL = 'https://server.fr/web-apps/apps/api/documents/api.js'
ONLYOFFICE_FILE_EXTENSION = ('doc', 'docx', 'ppt', 'pptx', 'xls', 'xlsx', 'odt', 'fodt', 'odp', 'fodp', 'ods', 'fods')
ONLYOFFICE_EDIT_FILE_EXTENSION = ('docx', 'pptx', 'xlsx')

Then restart the Seafile Server.