fix: detect architecture for golangci-lint download in Docker build

The golangci-lint binary was hardcoded as linux-amd64, causing Docker builds
to fail on arm64 hosts. The amd64 ELF binary cannot execute on aarch64,
producing a misleading shell syntax error during make check.

Use uname -m to detect the container architecture at build time and download
the matching binary. Both amd64 and arm64 SHA-256 hashes are pinned.

Closes #15

Dockerfile

@@ -13,10 +13,20 @@ RUN apk add --no-cache \
     curl
 
 # golangci-lint v2.10.1, 2026-02-25
-RUN curl -sSfL https://github.com/golangci/golangci-lint/releases/download/v2.10.1/golangci-lint-2.10.1-linux-amd64.tar.gz -o /tmp/golangci-lint.tar.gz && \
-    echo "dfa775874cf0561b404a02a8f4481fc69b28091da95aa697259820d429b09c99  /tmp/golangci-lint.tar.gz" | sha256sum -c - && \
+# SHA-256 checksums per architecture (amd64 / arm64)
+RUN set -e; \
+    ARCH="$(uname -m)"; \
+    if [ "$ARCH" = "aarch64" ] || [ "$ARCH" = "arm64" ]; then \
+      GOARCH="arm64"; \
+      HASH="6652b42ae02915eb2f9cb2a2e0cac99514c8eded8388d88ae3e06e1a52c00de8"; \
+    else \
+      GOARCH="amd64"; \
+      HASH="dfa775874cf0561b404a02a8f4481fc69b28091da95aa697259820d429b09c99"; \
+    fi; \
+    curl -sSfL "https://github.com/golangci/golangci-lint/releases/download/v2.10.1/golangci-lint-2.10.1-linux-${GOARCH}.tar.gz" -o /tmp/golangci-lint.tar.gz && \
+    echo "${HASH}  /tmp/golangci-lint.tar.gz" | sha256sum -c - && \
     tar -xzf /tmp/golangci-lint.tar.gz -C /tmp && \
-    mv /tmp/golangci-lint-2.10.1-linux-amd64/golangci-lint /usr/local/bin/ && \
+    mv "/tmp/golangci-lint-2.10.1-linux-${GOARCH}/golangci-lint" /usr/local/bin/ && \
     rm -rf /tmp/golangci-lint*
 
 WORKDIR /src