以下の構成で構築していたが、/__phpmyadmin/にアクセスできなかった。

docker-compose.yml

version: '3'

services:
  app:
    build:
      context: ./docker
      dockerfile: Dockerfile
    environment:
      APP_ENV: 'DOCKER_DEV'
    working_dir: /app
    volumes:
      - '.:/app'
    extra_hosts:
      - "host.docker.internal:host-gateway"

  proxy:
    image: steveltn/https-portal:1
    ports:
      - '80:80'
      - '443:443'
    environment:
      DOMAINS: 'domain.jp -> http://app, domain.com -> http://host.docker.internal:3000'
      STAGE: 'production'
    volumes:
      - ./docker/proxy/https-portal-data:/var/lib/https-portal
      - ./docker/proxy/nginx-conf/domain.com.ssl.conf.erb:/var/lib/nginx-conf/domain.com.ssl.conf.erb:ro
    depends_on:
      - app
    extra_hosts:
      - "host.docker.internal:host-gateway"

  phpmyadmin:
    image: 'phpmyadmin/phpmyadmin'
    environment:
      PMA_HOST: 'host.docker.internal'
      PMA_PORT: '3306'
      PMA_ABSOLUTE_URI: 'https://domain.com/__phpmyadmin/'
    extra_hosts:
      - "host.docker.internal:host-gateway"

docker/proxy/nginx-conf/domain.com.ssl.conf.erb

<%# 省略 %>
    location /__phpmyadmin/ {
        proxy_pass http://phpmyadmin;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}

https://domain.com/__phpmyadmin/にアクセスしても４０４となってしまっていた。

解決

rewrite ... を追加で解決した。

domain.com.ssl.conf.erb

<%# 省略 %>
    <%# https://github.com/phpmyadmin/docker/issues/210 %>
    location /__phpmyadmin/ {
        rewrite ^/__phpmyadmin(/.*)$ $1 break;
        proxy_pass http://phpmyadmin;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}

参考