Add documentation for reverse proxies (#4022)

Fixes #3904

docs/content/doc/usage/reverse-proxies.en-us.md

@@ -0,0 +1,102 @@
+---
+date: "2018-05-22T11:00:00+00:00"
+title: "Usage: Reverse Proxies"
+slug: "reverse-proxies"
+weight: 17
+toc: true
+draft: false
+menu:
+  sidebar:
+    parent: "usage"
+    name: "Reverse Proxies"
+    weight: 16
+    identifier: "reverse-proxies"
+---
+
+##  Using Nginx as a reverse proxy
+If you want Nginx to serve your Gitea instance you can the following `server` section inside the `http` section of `nginx.conf`:
+
+```
+server {
+    listen 80;
+    server_name git.example.com;
+
+    location / {
+        proxy_pass http://localhost:3000;
+    }
+}
+```
+
+## Using Nginx with a Sub-path as a reverse proxy
+
+In case you already have a site, and you want Gitea to share the domain name, you can setup Nginx to serve Gitea under a sub-path by adding the following `server` section inside the `http` section of `nginx.conf`:
+
+```
+server {
+    listen 80;
+    server_name git.example.com;
+
+    location /git/ { # Note: Trailing slash
+        proxy_pass http://localhost:3000/; # Note: Trailing slash
+    }
+}
+```
+
+Then set `[server] ROOT_URL = http://git.example.com/git/` in your configuration.
+
+## Using Apache HTTPD as a reverse proxy
+
+If you want Apache HTTPD to serve your Gitea instance you can add the following to you Apache HTTPD configuration (usually located at `/etc/apache2/httpd.conf` in Ubuntu):
+
+```
+<VirtualHost *:80>
+    ...
+    ProxyPreserveHost On
+    ProxyRequests off
+    ProxyPass / http://localhost:3000/
+    ProxyPassReverse / http://localhost:3000/
+</VirtualHost>
+```
+
+Note: The following Apache HTTPD mods must be enabled: `proxy`, `proxy_http`
+
+## Using Apache HTTPD with a Sub-path as a reverse proxy
+
+In case you already have a site, and you want Gitea to share the domain name, you can setup Apache HTTPD to serve Gitea under a sub-path by adding the following to you Apache HTTPD configuration (usually located at `/etc/apache2/httpd.conf` in Ubuntu):
+
+```
+<VirtualHost *:80>
+    ...
+    <Proxy *>
+         Order allow,deny
+         Allow from all
+    </Proxy>
+
+    ProxyPass /git http://localhost:3000 # Note: no trailing slash after either /git or port
+    ProxyPassReverse /git http://localhost:3000 # Note: no trailing slash after either /git or port
+</VirtualHost>
+```
+
+Then set `[server] ROOT_URL = http://git.example.com/git/` in your configuration.
+
+Note: The following Apache HTTPD mods must be enabled: `proxy`, `proxy_http`
+
+## Using Caddy with a Sub-path as a reverse proxy
+
+If you want Caddy to serve your Gitea instance you can add the following server block to your Caddyfile:
+
+```
+git.example.com {
+    proxy / http://localhost:3000
+}
+```
+
+##### How do I set up a sub-path with Caddy?
+
+In case you already have a site, and you want Gitea to share the domain name, you can setup Caddy to serve Gitea under a sub-path by adding the following to you server block in your Caddyfile:
+
+```
+git.example.com {
+    proxy /git/ http://localhost:3000 # Note: Trailing Slash after /git/
+}
+```