`devcontainer` fails to work with dockerimage's that use lowercase instructions

### Description
`devcontainer` cli fails to work with Dockerimages which uses lowercase instructions
Both, windows and linux environments are affected.

### Prerequirements
`devcontainer` cli version `0.26.1`

### Steps to reproduce
1. Create a folder
2. Create `.devcontainer/` folder
3. Add to `.devcontainer/` the `devcontainer.json` with the following content
```json
{
"dockerFile": "Dockerfile"
}
```
5. Add to `.devcontainer/` the `Dockerfile` with the following content
```Dockerfile
from alpine:latest
````
7. Run `devcontainer up --workfolder .`

### Expected result
No errors are generated

### Actual result
An error is generated

### Additional details
Here is a repo that demonstrates the bug
https://github.com/BusHero/devcontainer-bug-docker-lowercase-commands