Reverse proxying containerized angular app - docker

Question

We have an angular_app that upon running docker-compose up is accessible from
under localhost.
Now, we have another dockerized Nginx app that reverses proxies several apps. /app_A /app_B /agular_app. Now A and B work already but the angular_app gives 502.
A snippet from Nginx
location / {
proxy_pass http://agular_app:5001;
}
A snippet from the docker file
agular_app:
image: docker_repo/agular_app:latest
ports:
- 5001:80
The angular_app is accessible from localhost:5001
but not accessible from localhost (gives 502)
I have tried the following things:
Yes, changing 5001:5001, the dame 502
Different Nginx config such as adding proxy_set_header etc
Tried not using redirects over proxy_pass
For me... The missing puzzle piece is... Why reverse proxy does not work. Again, the dockerized angular image running manually (not containerized) on Nginx works fine.
Here is the docker file for angular_app
# Stage 1
FROM node:12-alpine3.12 as build-step
RUN mkdir -p /app
WORKDIR /app
COPY package.json /app
RUN npm install
COPY . /app
RUN npm run build --prod
# Stage 2
FROM nginx:1.14.2-alpine
COPY --from=build-step /app/frontend/dist /usr/share/nginx/html
COPY ./nginx_docker.conf /etc/nginx/conf.d/default.conf
EXPOSE 80
Error from Nginx log upon calling localhost:
gateway | 2021/05/27 06:52:07 [error] 28#28: *1 connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.1, server: localhost, request: "GET / HTTP/1.1", upstream: "http://172.18.0.2:5
001/", host: "localhost"
gateway | 2021/05/27 06:52:07 [error] 28#28: *1 connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.1, server: localhost, request: "GET / HTTP/1.1", upstream: "http://172.18.0.2:5
001/502.html", host: "localhost"
gateway | 172.18.0.1 - - [27/May/2021:06:52:07 +0000] "GET / HTTP/1.1" 502 552 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.212 Safari/537.36" "-"
gateway | 2021/05/27 06:52:07 [error] 28#28: *1 connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.1, server: localhost, request: "GET /favicon.ico HTTP/1.1", upstream: "http://1
72.18.0.2:5001/favicon.ico", host: "localhost", referrer: "http://localhost/"
gateway | 2021/05/27 06:52:07 [error] 28#28: *1 connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.1, server: localhost, request: "GET /favicon.ico HTTP/1.1", upstream: "http://1
72.18.0.2:5001/502.html", host: "localhost", referrer: "http://localhost/"