The inputs for /testcontainer: require new lines which is bad to set it from environment variables. Support for ; delimited string would be a nice addition.

${myEnvVarContainingListOfTestDLL}=mypath1\mytest.dll;mypath2\mytest2.dll

If we could input this into the MSTest runner plugin, this could also be used in conjecture of Inject environment variables from files if you already have those available.