Docker & Laravel : configure: error: Package requirements (oniguruma) were not met

What @kalatabe said is correct. But in case you absolutely wanted to make sure mbstring gets installed, you can also add libonig-dev to your apt-get install

Just remove mbstring from the docker-php-ext-install instruction.

The error is caused by a dependency problem - the mbstring extension requires the oniguruma library to make multibyte regular expression functions work. From the installation guide:

Oniguruma is necessary for the regular expression functions with multibyte character support. Oniguruma is bundled with mbstring. As of PHP 5.4.0, if Oniguruma is already installed on the system, --with-onig[=DIR] can be specified to use the installed library.

However, in the image that you're using, the extension is already installed and configured, so you don't need to do anything else:

$> docker run --rm -it php:7 php -r "var_dump(mb_ereg_match('^99.*', '123456'));"                                                                                      
bool(false)

$> docker run --rm -it php:7 php -r "var_dump(mb_ereg_match('^12.*', '123456'));"                                                                                      
bool(true)