add_dockerfile shloud optimise the package installation order

[ColinFay]

now it use the alphabetical order, but its not optmal during build.

for example :

RUN Rscript -e 'remotes::install_version("factoextra", version = "1.0.5")'
RUN Rscript -e 'remotes::install_version("FactoMineR", version = "1.42")'

will install twice FactoMineR, first time as factoextra dependencies (with an uncontroled version number), then in install_version("FactoMineR". and that's too bad :)

Migrated from ThinkR-open/golem#298