Net core generic repository pattern how to inject DbContext without knowing its type at compile time?

Lowest common factor here is DbContext.

Rafactor GenericRepository to explicitly depend on DbContext

public class GenericRepository<TEntity, TKey> : IRepository<TEntity, TKey>
    where TEntity : class, IEntity<TKey> {
    private readonly DbContext dbContext;

    public GenericRepository(DbContext dbContext) {
        this.dbContext = dbContext;
    }

    public IEnumerable<TEntity> GetAll() {
        return dbContext.Set<TEntity>().ToList();
    }
}

At composition root you would then make the association

services.AddDbContext<AppDbContext>(options =>
    options.UseSqlServer(Configuration["database:connectionString"]));

services.AddScoped(typeof(IRepository<,>), typeof(GenericRepository<,>));   
services.AddScoped<DbContext, AppDbContext>();

---

Update

In case of multiple Contexts, That would require a little more abstraction. In cases like that I create a specific abstraction for each context. like IDbContext or ILoggingContext

public interface IDbContext : IDisposable {
    int SaveContext();
    DbSet<TEntity> Set<TEntity>();
    //...other relevant EF members, etc
}

public interface IAppDbContext : IDbContext {

}

public interface ILogDbContext : IDbContext {

}

and have my DbContext derived classes inherit from the one relevant to it.

public class AppDbContext : DbContext, IAppDbContext {
    public AppDbContext(DbContextOptions<AppDbContext> dbContextOptions) : base(dbContextOptions) {
    }
}

public class LogDbContext : DbContext, ILogDbContext {
    public AppDbContext(DbContextOptions<LogDbContext> dbContextOptions) : base(dbContextOptions) {
    }
}

From there the generic repository would explicitly depend on the relevant abstraction(s)

public class GenericRepository<TEntity, TKey> : IRepository<TEntity, TKey>
    where TEntity : class, IEntity<TKey> {
    private readonly IDbContext dbContext;

    public GenericRepository(IAppDbContext dbContext) {
        this.dbContext = dbContext;
    }

    //...code removed for brevity
}

and then do the necessary configuration at composition root.

services.AddDbContext<AppDbContext>(options =>
    options.UseSqlServer(Configuration["database:appConnectionString"]));

services.AddDbContext<LogDbContext>(options =>
    options.UseSqlServer(Configuration["database:logConnectionString"]));

services.AddScoped(typeof(IRepository<,>), typeof(GenericRepository<,>));   
services.AddScoped<IAppDbContext, AppDbContext>();
services.AddScoped<ILogDbContext, LogDbContext>();