[deps] Fixed openssl dependency in Nginx Image #318

In #318, I initially removed the OpenSSL dependency from the Nginx
image, assuming it was included in the base image. However, the base
image removes the OpenSSL package in a later build step.
This commit reinstates the OpenSSL dependency to ensure proper
functionality.

Related to #318

images/openwisp_nginx/Dockerfile

@@ -1,6 +1,7 @@
 FROM nginx:1.27.0-alpine
 
 RUN apk add --update --no-cache \
+            openssl~=3.1.7-r0 \
             py3-pip~=23.3.1-r0 \
             certbot~=2.7.4-r0 \
             certbot-nginx~=2.7.4-r0 && \