Add Docker-based testing environment (#383)

Introduces reproducible testing infrastructure using Docker containers
to ensure consistent test execution across different development
environments.

Changes:
- Add Dockerfile.test for containerized test runs
- Add TESTING.md with comprehensive testing documentation
- Support for both local and Docker-based workflows

Benefits:
- Isolated test environment (Go 1.24-alpine)
- Layer caching for faster builds
- No dependency on local Go installation
- Easy CI/CD integration

Usage:
  docker build -f Dockerfile.test -t terraform-provider-civo:test .
  docker run --rm terraform-provider-civo:test

Author: DMajrekar

Dockerfile.test

@@ -0,0 +1,16 @@
+FROM golang:1.24-alpine
+
+WORKDIR /workspace
+
+# Install build dependencies
+RUN apk add --no-cache git make bash
+
+# Copy go.mod and go.sum first for better caching
+COPY go.mod go.sum ./
+RUN go mod download
+
+# Copy source code
+COPY . .
+
+# Default command runs unit tests
+CMD ["make", "test"]

TESTING.md

@@ -0,0 +1,97 @@
+# Testing Guide
+
+## Docker-Based Testing
+
+This project includes a Docker-based testing environment for reproducible test runs in an isolated environment.
+
+### Quick Start
+
+```bash
+# Build the test image
+docker build -f Dockerfile.test -t terraform-provider-civo:test .
+
+# Run unit tests
+docker run --rm terraform-provider-civo:test
+
+# Run build verification
+docker run --rm terraform-provider-civo:test make build
+
+# Run format check
+docker run --rm terraform-provider-civo:test make fmtcheck
+```
+
+### Test Environment
+
+- **Base Image**: golang:1.24-alpine
+- **Dependencies**: git, make, bash
+- **Go Modules**: Pre-downloaded for faster execution
+- **Default Command**: `make test` (runs unit test suite)
+
+### Benefits
+
+- **Reproducible**: Same environment for all developers and CI/CD
+- **Isolated**: No dependency on local Go installation or system packages
+- **Fast**: Layer caching speeds up subsequent builds
+- **Clean**: Tests run in a fresh environment every time
+
+### Local Testing (Without Docker)
+
+```bash
+# Run unit tests
+make test
+
+# Run acceptance tests (requires CIVO_TOKEN)
+TF_ACC=1 make testacc
+
+# Build the provider
+make build
+
+# Format code
+make fmt
+
+# Check formatting
+make fmtcheck
+```
+
+### CI/CD Integration
+
+The Docker test environment can be integrated into CI/CD pipelines:
+
+```yaml
+# Example GitHub Actions workflow
+test:
+  runs-on: ubuntu-latest
+  steps:
+    - uses: actions/checkout@v2
+    - name: Build test image
+      run: docker build -f Dockerfile.test -t terraform-provider-civo:test .
+    - name: Run tests
+      run: docker run --rm terraform-provider-civo:test
+```
+
+## Test Coverage
+
+The test suite covers:
+
+- Provider configuration and initialization
+- All resource CRUD operations (Create, Read, Update, Delete)
+- Data source queries
+- Input validation and error handling
+- SSH key generation and validation
+
+## Requirements
+
+- Docker (for containerized testing)
+- Go 1.24+ (for local testing)
+- CIVO API token (for acceptance tests only)
+
+## Contributing
+
+When adding new features:
+
+1. Add unit tests for new functionality
+2. Run the full test suite: `make test`
+3. Verify formatting: `make fmtcheck`
+4. Build verification: `make build`
+
+For acceptance tests that interact with the Civo API, follow existing patterns in the `civo/acceptance` directory.