Upgrade to Alpine 3.15

Dockerfile

@@ -15,7 +15,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #
-FROM php:8.0-fpm-alpine3.14
+FROM php:8.0-fpm-alpine3.15
 ADD https://github.com/mlocati/docker-php-extension-installer/releases/latest/download/install-php-extensions /usr/local/bin/
 RUN chmod uga+x /usr/local/bin/install-php-extensions && sync && \
     install-php-extensions zip ssh2 pdo_mysql mysqli