Add tzdata package in app docker image to allow use of TZ env var

In nginx container, it's already possible to choose the timezone by setting a TZ env var (because it's based on nginx:alpine image, that includes this package, see https://github.com/nginxinc/docker-nginx/blob/master/Dockerfile-alpine-slim.template#L96)

This commit simply adds this same package in app docker image.
It seems much cleaner to me than injecting /etc/timezone and /etc/localtime

app/Dockerfile

@@ -8,7 +8,7 @@ RUN apk add --no-cache dcron php81 php81-fpm php81-phar \
 	php81-mbstring php81-intl php81-xml php81-curl php81-simplexml \
 	php81-session php81-tokenizer php81-dom php81-fileinfo php81-ctype \
 	php81-json php81-iconv php81-pcntl php81-posix php81-zip php81-exif \
-	php81-openssl git postgresql-client sudo php81-pecl-xdebug rsync && \
+	php81-openssl git postgresql-client sudo php81-pecl-xdebug rsync tzdata && \
 	sed -i 's/\(memory_limit =\) 128M/\1 256M/' /etc/php81/php.ini && \
 	sed -i -e 's/^listen = 127.0.0.1:9000/listen = 9000/' \
 		-e 's/;\(clear_env\) = .*/\1 = no/i' \