A small problem in installing composer

stay docker In the mirror install composer

apt-get install composer

After installation perform composer -v

appear ; error

Usage:
  command [options] [arguments]

PHP Fatal error:  Uncaught Error: Call to undefined function Symfony\Component\Console\Helper\mb_detect_encoding() in /usr/share/php/Symfony/Component/Console/Helper/Helper.php:50
Stack trace:
#0 /usr/share/php/Symfony/Component/Console/Descriptor/TextDescriptor.php(329): Symfony\Component\Console\Helper\Helper::strlen()
#1 /usr/share/php/Symfony/Component/Console/Descriptor/TextDescriptor.php(97): Symfony\Component\Console\Descriptor\TextDescriptor->calculateTotalWidthForOptions()
#2 /usr/share/php/Symfony/Component/Console/Descriptor/TextDescriptor.php(197): Symfony\Component\Console\Descriptor\TextDescriptor->describeInputDefinition()
#3 /usr/share/php/Symfony/Component/Console/Descriptor/Descriptor.php(55): Symfony\Component\Console\Descriptor\TextDescriptor->describeApplication()
#4 /usr/share/php/Symfony/Component/Console/Helper/DescriptorHelper.php(67): Symfony\Component\Console\Descriptor\Descriptor->describe()
#5 /usr/share/php/Symfony/Component/Console/Command/ListCommand.php(75): Symfony\Component\Console\Helper\Des in /usr/share/php/Symfony/Component/Console/Helper/Helper.php on line 50

Fatal error: Uncaught Error: Call to undefined function Symfony\Component\Console\Helper\mb_detect_encoding() in /usr/share/php/Symfony/Component/Console/Helper/Helper.php:50
Stack trace:
#0 /usr/share/php/Symfony/Component/Console/Descriptor/TextDescriptor.php(329): Symfony\Component\Console\Helper\Helper::strlen()
#1 /usr/share/php/Symfony/Component/Console/Descriptor/TextDescriptor.php(97): Symfony\Component\Console\Descriptor\TextDescriptor->calculateTotalWidthForOptions()
#2 /usr/share/php/Symfony/Component/Console/Descriptor/TextDescriptor.php(197): Symfony\Component\Console\Descriptor\TextDescriptor->describeInputDefinition()
#3 /usr/share/php/Symfony/Component/Console/Descriptor/Descriptor.php(55): Symfony\Component\Console\Descriptor\TextDescriptor->describeApplication()
#4 /usr/share/php/Symfony/Component/Console/Helper/DescriptorHelper.php(67): Symfony\Component\Console\Descriptor\Descriptor->describe()
#5 /usr/share/php/Symfony/Component/Console/Command/ListCommand.php(75): Symfony\Component\Console\Helper\Des in /usr/share/php/Symfony/Component/Console/Helper/Helper.php on line 50

The lack of  php-mbstring

Command line installation

 apt-get install php-mbstring

Get it done