Acrescente os repositórios

echo "deb http://packages.dotdeb.org wheezy-php56 all" >> /etc/apt/sources.list
echo "deb-src http://packages.dotdeb.org wheezy-php56 all" >> /etc/apt/sources.list

baixe e instale as chaves GnuPG do repositório

wget http://www.dotdeb.org/dotdeb.gpg
apt-key add dotdeb.gpg

Atualize a lista de pacotes e instale o os pacotes php5 php5-mysql

apt-get update -q
apt-get install -yq php5 php5-mysql

Referências:
https://www.dotdeb.org/instructions/
https://hub.docker.com/r/ezegolub/apache-php56/~/dockerfile/