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

- Dockerfile: Pin golang and alpine base images to sha256 digests
- Dockerfile: Pin golangci-lint to v2.10.1 and goimports to v0.42.0
- CI workflows already pinned to commit SHAs (no changes needed)

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 golang.org/x/tools/cmd/goimports@latest
+RUN go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.10.1
+RUN go install golang.org/x/tools/cmd/goimports@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