Add Docker support for building and running the CLI tool

- Add DOCKER_HOST export to Makefile for remote Docker daemon
- Create multi-stage Dockerfile:
  - Build stage: golang:1.24-alpine with gcc, make, git
  - Runtime stage: alpine with ca-certificates, gnupg
  - Runs as non-root 'secret' user
- Add Makefile targets:
  - docker: build container as sneak/secret
  - docker-run: run container interactively
- Add .dockerignore to exclude build artifacts but keep .git
  for potential linker flags

Container includes GPG support for PGP unlockers and runs on Linux,
making it suitable for cross-platform testing and deployment.

Dockerfile

@@ -0,0 +1,50 @@
+# Build stage
+FROM golang:1.24-alpine AS builder
+
+# Install build dependencies
+RUN apk add --no-cache \
+    gcc \
+    musl-dev \
+    make \
+    git
+
+# Set working directory
+WORKDIR /build
+
+# Copy go mod files
+COPY go.mod go.sum ./
+
+# Download dependencies
+RUN go mod download
+
+# Copy source code
+COPY . .
+
+# Build the binary
+RUN CGO_ENABLED=1 go build -v -o secret cmd/secret/main.go
+
+# Runtime stage
+FROM alpine:latest
+
+# Install runtime dependencies
+RUN apk add --no-cache \
+    ca-certificates \
+    gnupg
+
+# Create non-root user
+RUN adduser -D -s /bin/sh secret
+
+# Copy binary from builder
+COPY --from=builder /build/secret /usr/local/bin/secret
+
+# Ensure binary is executable
+RUN chmod +x /usr/local/bin/secret
+
+# Switch to non-root user
+USER secret
+
+# Set working directory
+WORKDIR /home/secret
+
+# Set entrypoint
+ENTRYPOINT ["secret"]