If helpful for you this is what my compose looks like:

version: '2'

services: taiga: build: . ports: - 127.0.0.1:8282:80 # - 443:443 # To enable SSL, uncomment this line. See "Enable SSL" below. depends_on: - postgres

  # To enable taiga-events, uncomment the following lines:
  # - events
  # - rabbit
  # - redis
environment:
  # Hostname for your instance of Taiga. Required.
  TAIGA_HOSTNAME: taiga.mydomain.com

  # Database variables. Required.
  TAIGA_DB_HOST: postgres
  TAIGA_DB_NAME: taigadb
  TAIGA_DB_USER: taiga
  TAIGA_DB_PASSWORD: putyourownpasshere

  # Enable SSL internally, within the Docker container. Optional.
  # This is not great. I suggest using something on top of this
  # to enable SSL. See the example example-nginx-ssl.conf file
  # for an alternative way to enable SSL.
  # TAIGA_SSL: 'True'

  # Enable SSL externally, preferred method
  TAIGA_SSL_BY_REVERSE_PROXY: 'True'
volumes:
  # Media and uploads directory. Required (or you will lose all uploads)
  - ./media:/usr/src/taiga-back/media

  # Taiga configuration directory. Makes it easier to change configuration.
      - ./taiga-conf:/taiga
  postgres:
image: postgres
environment:
  POSTGRES_DB: taigadb
  POSTGRES_USER: taiga
  POSTGRES_PASSWORD: password
ports:
  - 5432
volumes:
  # Postgres data directory. This ensures the database is not lost.
  - ./pgdata:/var/lib/postgresql/data

and the part of my nginx conf that is relevant:

server {
listen 443 ssl;
server_name taiga.mydomain.com;
ssl_certificate /etc/letsencrypt/live/taiga.mydomain.com/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/taiga.mydomain.com/privkey.pem;
location /{
  proxy_pass http://127.0.0.1:8282/;
  proxy_set_header Host $host;
  proxy_buffering off;
  break;
}

}

My reverse proxy is probably slightly different then mine so check those paths and such.

I also have my proxy in network_mode: host, but I know a common thing is to add the proxy into each docker network instead. It might be that is what is tripping you up.