Allow to set custom upload size limit via NGINX_MAX_UPLOAD_SIZE env var in mwdb-web Docker image

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• [ ] I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

• we can't easily set client_max_body_size without building own image
• nginx Docker image provides correct start-web.sh on its own which is then unnecessarily overridden by our Dockerfile

What is the new behaviour?

• Added NGINX_MAX_UPLOAD_SIZE env reference to client_max_body_size option in nginx.conf.template
• Set correct path for nginx.conf.template (/etc/nginx/templates/default.conf.template instead of /etc/nginx/conf.d/default.conf.template) and added default 50M in ENV directive.

Test plan

• Check if setting works with no env value set and with custom value (I've done it during manual test of my changes)

Closing issues

closes #927