Docker tools timeout redux

[vladak]

#4247 (reply in thread) inspired me to allow command argument substitution to use environment variables as values.

While making the change, I realized that the command structure is broken - it needs to be a dictionary for stuff like resource limits and argument substitution to work. The per project workflow wiki will have to be adapted.

Command configuration in YAML now looks like this:

- command:
    args: [cmd, "--foo", "%FOO%"]
    limits: {RLIMIT_NOFILE: 1024}
    args_subst: {"%FOO%": "$FOO"}

The $ prefix in the $FOO value means that it will not be taken as literal; instead, the value of environment variable FOO will be taken.