fix: pin all external refs to cryptographic identity (closes #118)

- Pin Docker base images to sha256 digests (golang, alpine)
- Pin go install commands to commit SHAs (not version tags)
  - golangci-lint: 5d1e709b7be35cb2025444e19de266b056b7b7ee (v2.10.1)
  - goimports: 009367f5c17a8d4c45a961a3a509277190a9a6f0 (v0.42.0)
- CI workflow was already correctly pinned to commit SHAs

All references now use cryptographic identity, eliminating RCE risk
from mutable tags.

Dockerfile

@@ -1,11 +1,11 @@
 # Build stage
-FROM golang:1.25-alpine AS builder
+FROM golang@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder # golang:1.25-alpine
 
 RUN apk add --no-cache git make gcc musl-dev
 
 # Install golangci-lint v2
-RUN go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@latest
+RUN go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@5d1e709b7be35cb2025444e19de266b056b7b7ee # v2.10.1
-RUN go install golang.org/x/tools/cmd/goimports@latest
+RUN go install golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0 # v0.42.0
 
 WORKDIR /src
 COPY go.mod go.sum ./
@@ -20,7 +20,7 @@ RUN make check
 RUN make build
 
 # Runtime stage
-FROM alpine:3.19
+FROM alpine@sha256:6baf43584bcb78f2e5847d1de515f23499913ac9f12bdf834811a3145eb11ca1 # alpine:3.19
 
 RUN apk add --no-cache ca-certificates tzdata git openssh-client docker-cli