Fix: Correct GPG keyring path in Dockerfile.ci (fixes #99) (#101)

## Summary
Fix GPG keyring path inconsistency in Dockerfile.ci that caused Docker
build failures.

## Problem
The first `gpg --recv-keys` command was writing to
`/usr/share/keyrings/mono-official-archive-keyring.gpg` but the second
command was trying to export from `/tmp/mono-keyring.gpg`, causing the
export to fail.

## Solution
Use `/tmp/mono-keyring.gpg` consistently in both GPG commands:
1. Receive keys into `/tmp/mono-keyring.gpg`
2. Export from `/tmp/mono-keyring.gpg` to final destination

## Test plan
- [x] Fix applied to Dockerfile.ci
- [ ] Docker build workflow completes successfully
- [ ] Mono and .NET packages installed correctly

Fixes #99

Author: wkoszek

Dockerfile.ci

@@ -11,7 +11,7 @@ RUN apt-get update && apt-get install -y \
 
 # Add Mono repository and install Mono + .NET
 RUN gpg --homedir /tmp --no-default-keyring \
-    --keyring /usr/share/keyrings/mono-official-archive-keyring.gpg \
+    --keyring /tmp/mono-keyring.gpg \
     --keyserver hkp://keyserver.ubuntu.com:80 \
     --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF && \
     gpg --homedir /tmp --no-default-keyring \