POST requests to Nexus throught Nginx in a customized web context return error 400 POST is not supported

Question

I'm trying to setup Nexus 3 behind Nginx reverse proxy. Nexus and Nginx are in docker containers launched with docker-compose on a Centos 7.3 host. All docker images are the latest available.

Nexus listen on default port 8081 into its container. This port is exposed as 18081 on the docker host. Nexus is configured to be in /nexus web context.

Nginx listen on port 80 into its container which is also exposed on the docker host.

I just want to access the Nexus Repository Manager in a local Firefox with the address "localhost/nexus"

Here is the configuration:

docker-compose.yml:

version: '2'

networks:
  picnetwork:
    driver: bridge

services:
 nginx:
  image: nginx:latest
  restart: always
  hostname: nginx
  ports:
    - "80:80"
  networks:
    - picnetwork
  volumes:
    - /opt/cip/nginx:/etc/nginx/conf.d
  depends_on:
    - nexus

 nexus:
  image: sonatype/nexus3:latest
  restart: always
  hostname: nexus
  ports:
    - "18081:8081"
  networks:
    - picnetwork
  volumes:
    - /opt/cip/nexus:/nexus-data
  environment:
    - NEXUS_CONTEXT=nexus

Nginx default.conf (/opt/cip/nginx/default.conf in docker host which is /etc/nginx/conf.d/default.conf in Nginx container) :

proxy_send_timeout 120;
proxy_read_timeout 300;
proxy_buffering    off;
tcp_nodelay        on;

server {
    listen       80;
    server_name  localhost;

    client_max_body_size 1G;

    location /nexus {
        proxy_pass http://nexus:8081/nexus/;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}

The strange thing is that when web context is / (location /, proxy_pass without /nexux, NEXUS_CONTEXT=) it works fine, when the web context is /nexus (as configuration shown here) POST requests return "400 HTTP methop POST is not supported by this URL". But if I use "localhost:18081/nexus" in the second case it works fine.

Is that a Nexus bug, a Nginx bug, or am I missing something ?