ConnectionStringProvider.cs

public class ConnectionStringProvider
{
  private readonly IApplicationEnvironment _appEnv;
  private readonly IConfiguration _config;
  
  public ConnectionStringProvider(IApplicationEnvironment appEnv, IConfiguration config)
  {
    _appEnv = appEnv;
    _config = config;
  }
  
  public string GetConnectionString(string name)
  {
    return UpdateConnectionString(_config[$"Data:{name}"]);
  }
  
  private string UpdateConnectionString(string connectionString)
  {
    return (connectionString || "").Replace("|DataDirectory|", Path.Combine(_appEnv.ApplicationBasePath, ".data" + Path.DirectorySeparatorChar));
  }
}

databases.json

{
  "Data": {
    "Default": "SERVER=(localdb)\\MSSQLLocalD;AttachDbFileName=|DataDirectory|Database.mdf;Integrated Security=true;MultipleActiveResultSets=true"
  }
}

StartupSnippet.cs

services
  .AddEntityFramework()
  .AddSqlServer()
  .AddDbContext<MyDbContext>(b => b.UseSqlServer(provider.GetConnectionString("Default")));