feat: Add Docker support for easy distribution

- Multi-stage Dockerfile for minimal image size (~119MB)
- Includes PostgreSQL, MySQL, MariaDB client tools
- Non-root user (UID 1000) for security
- Docker Compose examples for all use cases
- Complete Docker documentation (DOCKER.md)
- Kubernetes CronJob examples
- Support for Docker secrets
- Multi-platform build support

Docker makes deployment trivial:
- No dependency installation needed
- Consistent environment
- Easy CI/CD integration
- Kubernetes-ready

Dockerfile

@@ -0,0 +1,58 @@
+# Multi-stage build for minimal image size
+FROM golang:1.24-alpine AS builder
+
+# Install build dependencies
+RUN apk add --no-cache git make
+
+WORKDIR /build
+
+# Copy go mod files
+COPY go.mod go.sum ./
+RUN go mod download
+
+# Copy source code
+COPY . .
+
+# Build binary
+RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -ldflags="-w -s" -o dbbackup .
+
+# Final stage - minimal runtime image
+FROM alpine:3.19
+
+# Install database client tools
+RUN apk add --no-cache \
+    postgresql-client \
+    mysql-client \
+    mariadb-client \
+    pigz \
+    pv \
+    ca-certificates \
+    tzdata
+
+# Create non-root user
+RUN addgroup -g 1000 dbbackup && \
+    adduser -D -u 1000 -G dbbackup dbbackup
+
+# Copy binary from builder
+COPY --from=builder /build/dbbackup /usr/local/bin/dbbackup
+RUN chmod +x /usr/local/bin/dbbackup
+
+# Create backup directory
+RUN mkdir -p /backups && chown dbbackup:dbbackup /backups
+
+# Set working directory
+WORKDIR /backups
+
+# Switch to non-root user
+USER dbbackup
+
+# Set entrypoint
+ENTRYPOINT ["/usr/local/bin/dbbackup"]
+
+# Default command shows help
+CMD ["--help"]
+
+# Labels
+LABEL maintainer="UUXO"
+LABEL version="1.0"
+LABEL description="Professional database backup tool for PostgreSQL, MySQL, and MariaDB"