docker: install mysqldump for backup

This commit installs mysqldump via the package mariadb-client to make
use of `php artisan backup:run`. Without that package the command fails
with the following error:

```
$ docker-compose exec app php artisan backup:run
Starting backup...
Dumping database pixelfed...
Backup failed because The dump process failed with exitcode 127 : Command not found : sh: 1: mysqldump: not found
```

contrib/docker/Dockerfile.apache

@@ -8,7 +8,7 @@ RUN apt-get update \
  && apt-get install -y --no-install-recommends git gosu \
       optipng pngquant jpegoptim gifsicle libpq-dev libsqlite3-dev locales zip unzip libzip-dev libcurl4-openssl-dev \
       libfreetype6 libicu-dev libjpeg62-turbo libpng16-16 libxpm4 libwebp6 libmagickwand-6.q16-6 \
-      libfreetype6-dev libjpeg62-turbo-dev libpng-dev libxpm-dev libwebp-dev libmagickwand-dev \
+      libfreetype6-dev libjpeg62-turbo-dev libpng-dev libxpm-dev libwebp-dev libmagickwand-dev mariadb-client\
  && sed -i '/en_US/s/^#//g' /etc/locale.gen \
  && locale-gen && update-locale \
  && docker-php-source extract \

contrib/docker/Dockerfile.fpm

@@ -8,7 +8,7 @@ RUN apt-get update \
  && apt-get install -y --no-install-recommends git gosu \
       optipng pngquant jpegoptim gifsicle libpq-dev libsqlite3-dev locales zip unzip libzip-dev libcurl4-openssl-dev \
       libfreetype6 libicu-dev libjpeg62-turbo libpng16-16 libxpm4 libwebp6 libmagickwand-6.q16-6 \
-      libfreetype6-dev libjpeg62-turbo-dev libpng-dev libxpm-dev libwebp-dev libmagickwand-dev \
+      libfreetype6-dev libjpeg62-turbo-dev libpng-dev libxpm-dev libwebp-dev libmagickwand-dev mariadb-client\
  && sed -i '/en_US/s/^#//g' /etc/locale.gen \
  && locale-gen && update-locale \
  && docker-php-source extract \