2 containers cannot talk to each other - docker

Question

I have 2 containers that need to talk to each other. the first container simulates sendmail daemon to mock out sending email. The second is localstack mocking out sending an email alert due do a cloud watch alarm. Apparently I am the only person in the whole world having this problem. The localstack people are at a complete loss on why I cannot have their pro-examples talk to SMTP4DEV.
OS = latest Mac OS Monterey 12.6
container 1 (smtp4dev to simulate sending email):
docker run --rm -it -p 3000:80 -p 2525:25 rnwood/smtp4dev
I have a python program that creates a mail message and when it sends. a mail message shows up in the smtp4dev container.
container 2:
export SMTP_HOST=host.docker.internal:2525
DEBUG=1 DNS_ADDRESS=127.0.0.1 LOCALSTACK_API_KEY=####### SMTP_HOST=host.docker.internal:2525 localstack start
when I run the code at https://github.com/localstack/localstack-pro-samples/tree/master/cloudwatch-metrics-aws. The log in the container is showing the it is trying to send a message to the SMTP4DEV container but fails.
both containers are in the bridge network. I would think that being in the same network they should be able to talk to each other
docker network inspect bridge
[
{
"Name": "bridge",
"Id": "150e446de01292139f0ff57b46cfbc1ab5b091ab589911cb88f3ee3abda983be",
"Created": "2022-10-11T18:37:05.724386042Z",
"Scope": "local",
"Driver": "bridge",
"EnableIPv6": false,
"IPAM": {
"Driver": "default",
"Options": null,
"Config": [
{
"Subnet": "172.17.0.0/16",
"Gateway": "172.17.0.1"
}
]
},
"Internal": false,
"Attachable": false,
"Ingress": false,
"ConfigFrom": {
"Network": ""
},
"ConfigOnly": false,
"Containers": {
"5be878e9738953a0e7047da51bc855c80c939d6c21bc5fbbedf61d4e2ddfb6e2": {
"Name": "localstack_main",
"EndpointID": "455353f471e1c9f80fcba66216c5c5face8a07bab9b6b66f9da39fecc221d5b4",
"MacAddress": "02:42:ac:11:00:03",
"IPv4Address": "172.17.0.3/16",
"IPv6Address": ""
},
"f16d5edf8b42286ac3484e307761c0bd63a49532f5f019321587c8a9588eb942": {
"Name": "goofy_haibt",
"EndpointID": "8ee0ab91dc3a3c308063b0da8e806c4b0a26a8297e6cc4549ba6dfd12d51a1fa",
"MacAddress": "02:42:ac:11:00:02",
"IPv4Address": "172.17.0.2/16",
"IPv6Address": ""
}
},
"Options": {
"com.docker.network.bridge.default_bridge": "true",
"com.docker.network.bridge.enable_icc": "true",
"com.docker.network.bridge.enable_ip_masquerade": "true",
"com.docker.network.bridge.host_binding_ipv4": "0.0.0.0",
"com.docker.network.bridge.name": "docker0",
"com.docker.network.driver.mtu": "1500"
},
"Labels": {}
}
]

Answer 1

I have a workaround. the option LAMBDA_DOCKER_NETWORK can be used to set the network from localstack.
DEBUG=1 DNS_ADDRESS=127.0.0.1 LOCALSTACK_API_KEY=dddddd SMTP_HOST=host.docker.internal:2525 LAMBDA_DOCKER_NETWORK=host localstack start
the real solution would be to use the "DOCKER_FLAGS" environment variable will the value of "--add-host=host.docker.internal:host-gateway". I am have asked LocalStack how I should tell use the host.docker.internal?