Usage instructions

Hello!

Maybe I used it wrong in my GitHub actions workflow, but the README file example did not seem to work for me, saying "apptainer-version: 1.3.6" did not exist. However, I was able to get it to work via the below, so I wanted to at least share it.

**_The README file:_**
```
steps:
- uses: actions/checkout@v2
- uses: eWaterCycle/setup-apptainer@v2
with:
apptainer-version: 1.3.6
- name: Run an apptainer container
run: apptainer run docker://alpine cat /etc/os-release
```

**_What I got to work_**
```
steps:
- name: Set Apptainer build pattern
uses: actions/checkout@v4
- uses: eWaterCycle/setup-apptainer@v2
with:
image: ghcr.io/apptainer/apptainer:latest
```

Thanks