How to install php-redis extension using the official PHP Docker image approach?

Redis is not an extension that is included in “php-src”, therefore you cannot use docker-php-ext-install. Use PECL:

RUN pecl install -o -f redis \
&&  rm -rf /tmp/pear \
&&  docker-php-ext-enable redis

On alpine php 7.3.5 we can use:

RUN apk add --no-cache pcre-dev $PHPIZE_DEPS \
        && pecl install redis \
        && docker-php-ext-enable redis.so

My opinion, the easiest way is:

RUN pecl install redis && docker-php-ext-enable redis

;)