I have error in the fifth line when asp.net try to connect db.
2023-06-02 09:36:59 warn: Microsoft.AspNetCore.DataProtection.Repositories.FileSystemXmlRepository[60]
2023-06-02 09:36:59       Storing keys in a directory '/root/.aspnet/DataProtection-Keys' that may not be persisted outside of the container. Protected data will be unavailable when container is destroyed.
2023-06-02 09:37:00 info: Microsoft.EntityFrameworkCore.Infrastructure[10403]
2023-06-02 09:37:00       Entity Framework Core 6.0.8 initialized 'AppIdentityDbContext' using provider 'Pomelo.EntityFrameworkCore.MySql:6.0.2' with options: ServerVersion 0.0-mysql 
2023-06-02 09:37:00 fail: Microsoft.EntityFrameworkCore.Database.Connection[20004]
2023-06-02 09:37:00       An error occurred using the connection to database '' on server 'db2'.
2023-06-02 09:37:00 Unhandled exception. System.InvalidOperationException: An exception has been raised that is likely due to a transient failure. Consider enabling transient error resiliency by adding 'EnableRetryOnFailure()' to the 'UseMySql' call.
2023-06-02 09:37:00  ---> MySqlConnector.MySqlException (0x80004005): Unable to connect to any of the specified MySQL hosts.
2023-06-02 09:37:00    at MySqlConnector.Core.ServerSession.ConnectAsync(ConnectionSettings cs, MySqlConnection connection, Int32 startTickCount, ILoadBalancer loadBalancer, IOBehavior ioBehavior, CancellationToken cancellationToken) in /_/src/MySqlConnector/Core/ServerSession.cs:line 433
2023-06-02 09:37:00    at MySqlConnector.MySqlConnection.CreateSessionAsync(ConnectionPool pool, Int32 startTickCount, Nullable`1 ioBehavior, CancellationToken cancellationToken) in /_/src/MySqlConnector/MySqlConnection.cs:line 926
2023-06-02 09:37:00    at MySqlConnector.MySqlConnection.OpenAsync(Nullable`1 ioBehavior, CancellationToken cancellationToken) in /_/src/MySqlConnector/MySqlConnection.cs:line 406
2023-06-02 09:37:00    at MySqlConnector.MySqlConnection.Open() in /_/src/MySqlConnector/MySqlConnection.cs:line 369
2023-06-02 09:37:00    at Microsoft.EntityFrameworkCore.Storage.RelationalConnection.OpenDbConnection(Boolean errorsExpected)
2023-06-02 09:37:00    at Microsoft.EntityFrameworkCore.Storage.RelationalConnection.OpenInternal(Boolean errorsExpected)
2023-06-02 09:37:00    at Microsoft.EntityFrameworkCore.Storage.RelationalConnection.Open(Boolean errorsExpected)
2023-06-02 09:37:00    at Pomelo.EntityFrameworkCore.MySql.Storage.Internal.MySqlRelationalConnection.Open(Boolean errorsExpected)
2023-06-02 09:37:00    at Pomelo.EntityFrameworkCore.MySql.Storage.Internal.MySqlDatabaseCreator.<>c__DisplayClass18_0.<Exists>b__0(DateTime giveUp)
2023-06-02 09:37:00    at Microsoft.EntityFrameworkCore.ExecutionStrategyExtensions.<>c__DisplayClass12_0`2.<Execute>b__0(DbContext c, TState s)
2023-06-02 09:37:00    at Pomelo.EntityFrameworkCore.MySql.Storage.Internal.MySqlExecutionStrategy.Execute[TState,TResult](TState state, Func`3 operation, Func`3 verifySucceeded)
2023-06-02 09:37:00    --- End of inner exception stack trace ---
2023-06-02 09:37:00    at Pomelo.EntityFrameworkCore.MySql.Storage.Internal.MySqlExecutionStrategy.Execute[TState,TResult](TState state, Func`3 operation, Func`3 verifySucceeded)
2023-06-02 09:37:00    at Microsoft.EntityFrameworkCore.ExecutionStrategyExtensions.Execute[TState,TResult](IExecutionStrategy strategy, TState state, Func`2 operation, Func`2 verifySucceeded)
2023-06-02 09:37:00    at Microsoft.EntityFrameworkCore.ExecutionStrategyExtensions.Execute[TState,TResult](IExecutionStrategy strategy, TState state, Func`2 operation)
2023-06-02 09:37:00    at Pomelo.EntityFrameworkCore.MySql.Storage.Internal.MySqlDatabaseCreator.Exists(Boolean retryOnNotExists)
2023-06-02 09:37:00    at Pomelo.EntityFrameworkCore.MySql.Storage.Internal.MySqlDatabaseCreator.Exists()
2023-06-02 09:37:00    at Microsoft.EntityFrameworkCore.Migrations.HistoryRepository.Exists()
2023-06-02 09:37:00    at Microsoft.EntityFrameworkCore.Migrations.HistoryRepository.GetAppliedMigrations()
2023-06-02 09:37:00    at Microsoft.EntityFrameworkCore.RelationalDatabaseFacadeExtensions.GetAppliedMigrations(DatabaseFacade databaseFacade)
2023-06-02 09:37:00    at Microsoft.EntityFrameworkCore.RelationalDatabaseFacadeExtensions.GetPendingMigrations(DatabaseFacade databaseFacade)
2023-06-02 09:37:00    at ClothingShop.Models.IdentitySeedData.EnsurePopulated(IApplicationBuilder app) in /src/Models/IdentitySeedData.cs:line 16
2023-06-02 09:37:00    at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_1(Object state)
2023-06-02 09:37:00    at System.Threading.QueueUserWorkItemCallbackDefaultContext.Execute()
2023-06-02 09:37:00    at System.Threading.ThreadPoolWorkQueue.Dispatch()
2023-06-02 09:37:00    at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()
2023-06-02 09:37:00    at System.Threading.Thread.StartCallback()

I have an asp.net core project and two databases. I am creating a docker compose file.
version: '3.8'

services:
  web:
    build: .
    ports:
      - "8000:80"
    depends_on:
      - db1
      - db2

  db1:
    image: mysql:8.0.30
    environment:
      MYSQL_ROOT_PASSWORD: root
      MYSQL_DATABASE: clothingshop1
    volumes:
      - ./db1:/docker-entrypoint-initdb.d
    ports:
      - "3307:3306"

  db2:
    image: mysql:8.0.30
    environment:
      MYSQL_ROOT_PASSWORD: root
      MYSQL_DATABASE: identity
    volumes:
      - ./db2:/docker-entrypoint-initdb.d
    ports:
      - "3308:3306"

I am using generated file docker
#See https://aka.ms/customizecontainer to learn how to customize your debug container and how Visual Studio uses this Dockerfile to build your images for faster debugging.

FROM mcr.microsoft.com/dotnet/aspnet:6.0 AS base
WORKDIR /app
EXPOSE 80
EXPOSE 443

FROM mcr.microsoft.com/dotnet/sdk:6.0 AS build
WORKDIR /src
COPY ["ClothingShop.csproj", "."]
RUN dotnet restore "./ClothingShop.csproj"
COPY . .
WORKDIR "/src/."
RUN dotnet build "ClothingShop.csproj" -c Release -o /app/build

FROM build AS publish
RUN dotnet publish "ClothingShop.csproj" -c Release -o /app/publish /p:UseAppHost=false

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "ClothingShop.dll"]

In programm.cs i have:
builder.Services.AddDbContext<StoreDbContext>(opts =>
{
    opts.UseMySql(builder.Configuration.GetConnectionString("ClothingShopConnection"), new MySqlServerVersion(new Version()), b => b.EnableRetryOnFailure());
});

and
builder.Services.AddDbContext<AppIdentityDbContext>(options =>
 options.UseMySql(builder.Configuration.GetConnectionString("IdentityConnection"), new MySqlServerVersion(new Version())));

In appsettings.json i have:
  "AllowedHosts": "*",
  "ConnectionStrings": {
    "ClothingShopConnection": "server=db1;port=3307;user=root;password=root;Pooling=true;Max Pool Size=200;database=clothingshop1",
    "IdentityConnection": "server=db2;port=3308;user=root;password=root;database=identity"
  },

I run
docker-compose up -d in cmd. And i have this result Created container but asp won't start.
I try edit appsettings.json to:
    "ClothingShopConnection": "server=localhost;port=3306;user=root;password=root;Pooling=true;Max Pool Size=200;database=clothingshop1",
    "IdentityConnection": "server=localhost;port=3306;user=root;password=root;database=identity"

    "ClothingShopConnection": "server=localhost;port=3307;user=root;password=root;Pooling=true;Max Pool Size=200;database=clothingshop1",
    "IdentityConnection": "server=localhost;port=3308;user=root;password=root;database=identity"

    "ClothingShopConnection": "server=db-1;port=3307;user=root;password=root;Pooling=true;Max Pool Size=200;database=clothingshop1",
    "IdentityConnection": "server=db-2;port=3308;user=root;password=root;database=identity"

but to no avail.
An interesting fact is that it mysql worckbanch connector sees these databases and makes it possible to connect to them through localhost: 3307 and 08, respectively.Successful connection. Db created correct.
I've looked at similar threads but haven't found an answer.

ASP.Net Core can't connect to MySql database in docker
I solved my problem by configuring the connection string like this:
"ConnectionStrings": {
"ClothingShopConnection": "server=dbClothingShop;port=3306;user=root;password=root;Pooling=true;Max Pool Size=200;database=clothingshop1",
"IdentityConnection": "server=dbIdentity;port=3306;user=root;password=root;database=identity"
}

And apparently part of the problem was due to incorrect reading of tables from the database. Tables with capital letters were expected, but in the table they are with small letters (apparently something broke when importing and exporting the database).
I solved it with attributes DataAnnotation:
[Table("size")]
public class Size
{
   //SizeID...
}

Thanks a lot for the great questions @QiangFu
