Every container has its own network namespace, so every container can listen on port 80. The host has one set of ports. That is the whole rule:

Same container port, different host ports

host :8080 ──► blue  :80  nginx
host :8081 ──► green :80  nginx

docker run -d --name blue -p 8080:80 nginx:1.27
docker run -d --name green -p 8081:80 nginx:1.27

A host port can be used once

docker run -d --name red -p 8080:80 nginx:1.27

docker: Error response from daemon: driver failed programming external connectivity on endpoint red (f204b401e44f...): Bind for 0.0.0.0:8080 failed: port is already allocated

The container was created but could not start. docker ps -a lists it as Created. Remove it before reusing the name:

CONTAINER ID   IMAGE        COMMAND                  CREATED         STATUS         PORTS                  NAMES
6df65503b990   nginx:1.27   "/docker-entrypoint.…"   2 seconds ago   Created                               red

Choose the host address

-p takes an optional host address in front:

docker run -d --name admin -p 127.0.0.1:8082:80 httpd:2.4
docker port admin

80/tcp -> 127.0.0.1:8082

Use 127.0.0.1 for admin pages and databases that only local tools should reach.

Let Docker pick: -P

-P (capital) publishes every port the image declares with EXPOSE, each on a free high host port:

docker run -d --name auto -P nginx:1.27
docker port auto

80/tcp -> 0.0.0.0:32768

Read the chosen port with docker port or docker ps. It changes each time the container is created.

Two errors, two causes

-p 8090:8080 nginx      host :8090 ──► container :8080   (nginx is on :80)

The second error means the right side of -p is wrong. Check which port the program listens on (its docs, docker logs, or EXPOSE in docker image inspect) and recreate the container.