Fixes #1202: Added an available memory check to constrain the number of CPU c…

[sanchayanghosh]

…ores used, and moved fetch.sh call inside a dockerfile, to allowe building inside docker without having to install cmake externally.

Description

A fix for #1202 - Here, I put a pre-condition check on the available memory on the system.

This will usually be the memory available to the individual docker. However, since we are using an overlay system, with no specific memory caps/ caps on CPU shares, we should be seeing the state of the entire system.

Features list

Fix #1202 - AliceVision would crash the linux environment because it exceeds the available memory when building.

Implementation remarks

During the actual build, I found that putting 2 GB per job was ideal. 1GB per job would crash the system.

In the worst case, we have an nproc/2 situation, which anyhow is the default behaviour of cmake.