Nginx caching when serving static content - linux

Question

I'm serving a directory statically with Nginx, using the following site config, and works well:
server {
listen 80;
server_name spa.me.com;
access_log /var/log/nginx/spa-access.log;
error_log /var/log/nginx/spa-error.log;
location / {
autoindex on;
root /home/ubuntu/spa/dist/;
}
}
Now I want to add caching feature to this server. So I've read some posts and:
Declare a proxy_cache_path.
Use it from my server.
Add a X-Cached header to tell the client the cache status.
So now my config is as follows:
proxy_cache_path /var/cache/nginx/spa levels=1:2 keys_zone=spa:10m inactive=15m;
server {
listen 80;
server_name spa.mediasmart.mobi;
access_log /var/log/nginx/spa-access.log;
error_log /var/log/nginx/spa-error.log;
location / {
proxy_cache spa;
add_header X-Cached $upstream_cache_status;
autoindex on;
root /home/ubuntu/spa/dist/;
}
}
For some reason, the X-Cache header is never sent, so I'm not able to know (using curl, for instance) if a resource is recovered from the cache or not. Furthermore, the cache directory is empty, so it seems no caching feature has been really enabled.
I've also tried to add the $upstream_cache_status to my logs file, creating a custom format and using it:
...
log_format spa_log_format '$remote_addr - $upstream_cache_status [$time_local] "$request" $status $body_bytes_sent "$http_referer" "$http_user_agent"';
...
server {
...
access_log /var/log/nginx/spa-access.log spa_log_format;
...
}
But $upstream_cache_status never has a value. Always an empty string is printed on its place in the log.
What I'm doing wrong? Thanks