It does work in fly.toml, actually, although there are some rough edges that might cause a person to conclude otherwise, 
.
Here is a full example of a user process running as PID 1…
app = "multiverse"
primary_region = "ewr"
swap_size_mb = 512
machine_config = "solo-config.json"
[build]
image = "debian:bookworm"
And the JSON file:
{
"containers": [
{"name": "c1",
"image": "debian:bookworm",
"cmd": ["sleep", "inf"]}
]
}
Then…
$ fly app create multiverse
$ fly config validate --strict
$ fly deploy --ha=false --depot=false
$ fly ssh console
# od -c /proc/1/cmdline
0000000 s l e e p \0 i n f \0
0000012
I.e., PID 1 is the cmd from solo-config.json.
Aside: The build.image knob is mandatory, but it’s not clear what it really changes at run time…