fix: Docker build fails with no space left on device

Remove redundant 'go mod tidy' from Dockerfile — go.mod/go.sum are
already committed tidy, so 'go mod download' + 'go build' is sufficient.
The tidy step downloaded huge transitive test deps (gonum 500MB+ via
OpenTelemetry → gRPC chain) that exhausted Docker's disk.

Add .dockerignore to exclude Rust target/ (521MB), .git/, and test
directories from the build context, further reducing image build size
and preventing COPY disk exhaustion.

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

Author: lpcox

.dockerignore

@@ -0,0 +1,22 @@
+# Rust build artifacts (500MB+)
+guards/github-guard/rust-guard/target/
+
+# Test directories
+test/
+.github/
+
+# Git metadata
+.git/
+
+# Build artifacts
+awmg
+mcpg
+*.test
+*.out
+coverage.*
+
+# Runtime/dev files
+.env
+.env.*
+.DS_Store
+.serena/

Dockerfile

@@ -9,7 +9,6 @@ RUN go mod download
 
 # Copy source code
 COPY . .
-RUN go mod tidy
 
 # Build argument for version (defaults to "dev")
 ARG VERSION=dev