ci: install protoc in Docker CICD workflow

The docker.yml workflow was missing protobuf-compiler, causing
build.rs (tonic_build for proto/pipe.proto) to fail on the
self-hosted runner.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: vsilent

.github/workflows/docker.yml

@@ -27,11 +27,11 @@ jobs:
         with:
           ref: ${{ github.ref }}
 
-      - name: Install OpenSSL build deps
+      - name: Install OpenSSL and protoc build deps
         if: runner.os == 'Linux'
         run: |
           sudo apt-get update
-          sudo apt-get install -y pkg-config libssl-dev
+          sudo apt-get install -y pkg-config libssl-dev protobuf-compiler
 
       - name: Verify .sqlx cache exists
         run: |