Composer: request PHP extention mongodb

Build Fail when installing mongodb:

```
Your requirements could not be resolved to an installable set of packages.
Problem 1
- Installation request for mongodb/mongodb 1.7.1 -> satisfiable by mongodb/mongodb[1.7.1].
- mongodb/mongodb 1.7.1 requires ext-mongodb ^1.8 -> the requested PHP extension mongodb is missing from your system.
Problem 2
- mongodb/mongodb 1.7.1 requires ext-mongodb ^1.8 -> the requested PHP extension mongodb is missing from your system.
- jenssegers/mongodb v3.6.5 requires mongodb/mongodb ^1.4 -> satisfiable by mongodb/mongodb[1.7.1].
- Installation request for jenssegers/mongodb v3.6.5 -> satisfiable by jenssegers/mongodb[v3.6.5].

To enable extensions, verify that they are enabled in your .ini files:
- /usr/local/etc/php/php-cli.ini
- /usr/local/etc/php/conf.d/docker-php-ext-sodium.ini
- /usr/local/etc/php/conf.d/docker-php-ext-zip.ini
You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.
```

## Affected builder image

`gcr.io/cloud-builders-community/composer`

## Expected Behavior
Composer build step success
## Actual Behavior
Composer build step fail
## Steps to Reproduce the Problem

1. compser.json
```
{
"name": "laravel/lumen",
"description": "The Laravel Lumen Framework.",
"keywords": ["framework", "laravel", "lumen"],
"license": "MIT",
"type": "project",
"require": {
. . .
"mongodb/mongodb": "^1.7",
. . .
},
"config": {
"preferred-install": "dist",
"sort-packages": true,
"optimize-autoloader": true
},
"minimum-stability": "dev",
"prefer-stable": true
}
```

## Additional Info