podman: make env.MachineDirs configurable to have proper separation between podman-machine and macadam

[cfergeau]

At the moment, podman machine ... can be used to interact with VMs created by macadam. This is not really desirable as the macadam VMs can be different from what podman machine expects.
We should give their own directories to macadam owned VMs, maybe changing env.MachineDirs and similar things will be enough for this.

[lstocchi]

My idea was to allow costumization through env variables so to easily revert to podman original paths if needed.
An example is the config directory which is currently calculated using a simple join with static values filepath.Join(conf, "containers", "podman", "machine")

Initially i wanted to do something like

machineDataDir := os.Getenv("MACADAM_MACHINE_DATA")
if machineDataDir == "" {
	machineDataDir = filepath.Join("macadam", "machine")
}
confDir := filepath.Join(conf, machineDataDir)

However, the "containers" subfolder is used in many placed, like the external podman storage dependency.

Maybe we should keep it as mandatory.
So to have

confDir := filepath.Join(conf, "containers", machineDataDir)

[cfergeau]

I think most of the directory logic is in https://github.com/containers/podman/tree/main/pkg/machine/env which is where you were thinking of putting the MACADAM_MACHINE_DATA resolution?

However, the "containers" subfolder is used in many placed, like the external podman storage dependency.

Note that we don't really have a use for this in macadam (do we?), if we try to access it that would be a bug.

[cfergeau]

Maybe overriding DataDirPrefix and ConfDirPrefix would be enough for what we want?

[lstocchi]

@cfergeau yes correct it is mostly in https://github.com/containers/podman/tree/main/pkg/machine/env .
Pushed a PR with the env variables cfergeau/podman#1

Ok, tbh i don't even know when the storage dep comes in play. Maybe when adding volumes? I just saw that it was using a similar path for the storage folder

[lstocchi]

Maybe overriding DataDirPrefix and ConfDirPrefix would be enough for what we want?

We should also overridden the runtime dir but it's a private func so maybe with the env variables we can have the same behavior for all dirs