Allow `--build-command` to be passed for docker builds

**What is the problem this feature would solve? Please describe.**

We're trying to improve the performance of our Docker based builds for DOAP, and was looking into Depot (https://depot.dev), which is a drop-in replacement for Docker. So `docker build` -> `depot build`.

Unfortunately the `doctl app dev build` command doesn't support a custom build command for docker builds.

```
✘ build command overrides are ignored for dockerfile-based builds
```

**Describe the solution you'd like**

It would be nice if we could substitute `docker` for `depot`. Whether it's something like `--build-command 'depot build'` or a completely unique option like `--use-depot`.

**Additional context**