sandbox openssh-server (#196)

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> [!IMPORTANT]
> Remove sandbox Dockerfile and YAML, add `openssh-server` to default
Dockerfile.
> 
>   - **Dockerfile Changes**:
> - Deletes `.ellipsis/sandbox.Dockerfile` and `.ellipsis/sandbox.yaml`,
removing the sandbox configuration.
> - Adds `openssh-server` installation to
`.ellipsis/sandboxes/default.Dockerfile`.
> 
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Author: nsbradford

.ellipsis/sandbox.Dockerfile

@@ -9,7 +9,8 @@ RUN apt-get update && apt-get install -y \
     python3 \
     make \
     g++ \
-    git
+    git \
+    openssh-server
 
 # https://docs.doppler.com/docs/install-cli
 RUN apt-get update && apt-get install -y apt-transport-https ca-certificates curl gnupg && \