dockerfile: updated dockerfile with valgrind installation

[StarTrooper08]

updated dockerfile with valgrind installation

PR for issue number : #101

[hturner]

It seems that valgrind is already installed (this is on a codespace on the main branch):

@hturner ➜ /workspaces/r-dev-env $ dpkg -l valgrind
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name           Version           Architecture Description
+++-==============-=================-============-=============================================================
ii  valgrind       1:3.18.1-1ubuntu2 amd64        instrumentation framework for building dynamic analysis tools

I thought we may also need valgrind-devel, but it seems this does not exist on Ubuntu and the valgrind package already has the header (.h) files (https://packages.ubuntu.com/focal/amd64/valgrind/filelist). So we can close this PR and I will update the issue to make it a documentation one.