`BUILD_CORES_MAKE` > 1 gives occasional installation errors

[milanmlft]

omopcat/.env.sample

Line 14 in 65523d3

BUILD_CORES_MAKE=4 # Number of cores to use for make in docker build

Was added in #118 to speed up installation.
Unfortunately, this sometimes results in installation errors in the Docker images. Unsure what the cause is but assuming there could be some weird interactions when installing R packages from source.

Setting BUILD_CORES_MAKE=1 resolved the installation issues for me.