TL;DR: Move uid/gid off the long syntax. Use `--tmpfs /secrets:uid=1001,gid=1001,mode=0700` (or the HostConfig.Tmpfs API field) instead of `--mount type=tmpfs` with uid/gid options. The daemon only implements uid/gid for tmpfs on the legacy short-syntax path; the Mounts API rejects them as invalid options.

## The error

```text
invalid mount config for type "tmpfs": invalid option: uid
```

## Fix it

1. Switch to the short syntax (CLI):
   `docker run --tmpfs /secrets:uid=1001,gid=1001,mode=0700,size=16m [image]`
   Expected: container starts; `mount | grep /secrets` shows uid=1001,gid=1001.
2. In the Engine API or a Docker SDK, put the entry in `HostConfig.Tmpfs` instead of `Mounts`:
   `{"HostConfig": {"Tmpfs": {"/secrets": "uid=1001,gid=1001,mode=0700,size=16m"}}}`
   Expected: create returns 201 instead of the mount-config error.
3. In compose, use the short list form:
   `tmpfs: - /secrets:uid=1001,gid=1001,mode=0700`
   Expected: `docker compose up` starts the service.

## When this applies
- Container create through the Engine API/SDK with mount.Mount{Type: tmpfs, TmpfsOptions.Options: [[uid,...],[gid,...]]}
- `--mount type=tmpfs` long syntax carrying uid/gid

## When this does NOT apply
- Plain `--tmpfs /path` with no options (always works)
- `tmpfs-size` / `tmpfs-mode` in the long syntax (those are valid there)
- tmpfs filling up at runtime ("no space left" is a different problem)

## Versions
Docker Engine 20.10 through 29.x (reproduced on 29.3.0).

## Why it happens
The daemon implements uid/gid/mode/size for tmpfs only through the legacy HostConfig.Tmpfs option-string path. The newer Mounts API TmpfsOptions accepts a smaller option set, so uid/gid passed that way are rejected during mount validation, before the container is created.

## Edge cases
- `mode` works in both syntaxes; only uid/gid are rejected via the Mounts API.
- Short-syntax size accepts m/g suffixes (`size=16m`); long-syntax `tmpfs-size` wants raw bytes.
- tmpfs is RAM-backed and non-persistent; do not put data you need after restart there.