Make pytz dep explicit and update alpine

docker/ntpd/Dockerfile

@@ -17,7 +17,7 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-FROM alpine:3.12
+FROM alpine:3.15
 
 RUN apk update && apk add chrony && \
     chown -R chrony:chrony /var/lib/chrony

requirements.txt

@@ -1,3 +1,4 @@
+pytz
 Django
 djangorestframework
 django-health-check