phpiredis is not available in Docker image?

I have set the necessary environmental variables:

```
SYMFONY__ENV__REDIS_HOST
SYMFONY__ENV__REDIS_PORT
SYMFONY__ENV__REDIS_SCHEME
SYMFONY__ENV__REDIS_PASSWORD
```

But when I got to "Import / Wallabag v2" (`/import/wallabag-v2`) I have the following issue:

```
500: Internal Server Error

The "phpiredis" extension is required by this connection backend.
```

---

I don't see it in `php -me` output:
```
/ # php -me
[PHP Modules]
amqp
bcmath
Core
ctype
curl
date
dom
filter
gd
gettext
hash
iconv
intl
json
libxml
mbstring
mysqlnd
openssl
pcre
PDO
pdo_mysql
pdo_pgsql
pdo_sqlite
Phar
readline
Reflection
session
SimpleXML
sockets
SPL
standard
tidy
tokenizer
xml
xmlreader
zlib

[Zend Modules]
```

Is `phpiredis` missing from [Dockerfile](https://github.com/wallabag/docker/blob/master/Dockerfile)?