Hopefully this will help someone. This seems to work for me. Subscribed communities update, I am able to post. I'm the only user right now on my server. NPM took me a bit of messing around with the config but I think I have everything working, some of this may be redundant / non functional but I don't have the will to go line by line to see what more I can take out. Here is how I have it configured. Note that some things go to the Lemmy UI port and some to the Lemmy port. These should be defined in your docker-compose if you're using that. (Mine is below)

On the first tab in NPM, "Details" I have the following:

Scheme: http
Hostname: <docker ip>
Port: <lemmy-ui port>
Block Common Exploits and Websockets Support are enabled.

On the Custom Locations page, I added 4 locations, you have to do one for each directory even though the ip/ports are the same.

Location: /api
Scheme: http
Hostname: <docker ip>
Port: <lemmy port>

Repeat the above for "/feeds", "/pictrs", and "/nodeinfo". The example file they give also says to have ".well_known" in there but as far as I know that's just for Let's Encrypt which NPM should be handling for us.

On the SSL tab, I have a Let's Encrypt certificate set up. Force SSL, HTTP/2 Support, and HSTS Enabled.

On the Advanced tab, I have the following:

 location / {

   set $proxpass "http://<docker ip>:<lemmy-ui port>";
   if ($http_accept = "application/activity+json") {

     set $proxpass "http://<docker ip>:<lemmy-ui port>";`
   }
   if ($http_accept = "application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\"") {
     set $proxpass "http://<docker ip>:<lemmy-ui port>";
   }
   if ($request_method = POST) {
     set $proxpass "http://<docker ip>:<lemmy-ui port>";
   }
   proxy_pass $proxpass;
   
   rewrite ^(.+)/+$ $1 permanent;
    # Send actual client IP upstream
   proxy_set_header X-Real-IP $remote_addr;
   proxy_set_header Host $host;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
 }

I probably should add in my docker compose file as well... I'm far from a docker expert. This is reasonably close to their examples and others I found. I removed nginx from in here since we already have a proxy. I disabled all the debug logging because it was using disk space. I also removed all the networking lines because I'm not smart enough to figure it all out right now. If you use this, look out for the < > sections, you need to set your own domain/hostname, and postgres user/password.

version: "3.3"

services:
  lemmy:
    image: dessalines/lemmy:0.17.3
    hostname: lemmy
    restart: always
    ports:
      - 8536:8536
    environment:
      - RUST_LOG="warn"
      - RUST_BACKTRACE=full
    volumes:
      - ./lemmy.hjson:/config/config.hjson:Z
    depends_on:
      - postgres
      - pictrs

  lemmy-ui:
    image: dessalines/lemmy-ui:0.17.4
    # use this to build your local lemmy ui image for development
    # run docker compose up --build
    # assuming lemmy-ui is cloned besides lemmy directory
    # build:
    #   context: ../../lemmy-ui
    #   dockerfile: dev.dockerfile
    ports:
      - 1234:1234
    environment:
      # this needs to match the hostname defined in the lemmy service
      - LEMMY_UI_LEMMY_INTERNAL_HOST=lemmy:8536
      # set the outside hostname here
      - LEMMY_UI_LEMMY_EXTERNAL_HOST=< domain name>
      - LEMMY_HTTPS=false
      - LEMMY_UI_DEBUG=true
    depends_on:
      - lemmy
    restart: always

  pictrs:
    image: asonix/pictrs:0.4.0-beta.19
    # this needs to match the pictrs url in lemmy.hjson
    hostname: pictrs
    # we can set options to pictrs like this, here we set max. image size and forced format for conversion
    # entrypoint: /sbin/tini -- /usr/local/bin/pict-rs -p /mnt -m 4 --image-format webp
    environment:
      - PICTRS_OPENTELEMETRY_URL=http://otel:4137
      - PICTRS__API_KEY=API_KEY
      - RUST_LOG=debug
      - RUST_BACKTRACE=full
      - PICTRS__MEDIA__VIDEO_CODEC=vp9
      - PICTRS__MEDIA__GIF__MAX_WIDTH=256
      - PICTRS__MEDIA__GIF__MAX_HEIGHT=256
      - PICTRS__MEDIA__GIF__MAX_AREA=65536
      - PICTRS__MEDIA__GIF__MAX_FRAME_COUNT=400
    user: 991:991
    volumes:
      - ./volumes/pictrs:/mnt:Z
    restart: always

  postgres:
    image: postgres:15-alpine
    # this needs to match the database host in lemmy.hson
    # Tune your settings via
    # https://pgtune.leopard.in.ua/#/
    # You can use this technique to add them here
    # https://stackoverflow.com/a/30850095/1655478
    hostname: postgres
    command:
      [
        "postgres",
        "-c",
        "session_preload_libraries=auto_explain",
        "-c",
        "auto_explain.log_min_duration=5ms",
        "-c",
        "auto_explain.log_analyze=true",
        "-c",
        "track_activity_query_size=1048576",
      ]
    ports:
      # use a different port so it doesnt conflict with potential postgres db running on the host
      - "5433:5432"
    environment:
      - POSTGRES_USER=< dbuser >
      - POSTGRES_PASSWORD=< dbpassword>
      - POSTGRES_DB=lemmy
    volumes:
      - ./volumes/postgres:/var/lib/postgresql/data:Z
    restart: always

There's another post over here https://fernchat.esotericmonkey.com/post/277 with a similar setup, less stuff in the Advanced tab. If I'm bad at explaining maybe another guide will help.