Fix Dockerfile for Guestbook Application Build

[AtharvaNawathe]

This PR updates the Dockerfile to resolve build errors caused by outdated dependencies.

Issue Addressed:

• The previous Dockerfile using Go 1.10.0 failed to build due to incompatibilities with newer Go packages, specifically the undefined: any error in github.com/gorilla/mux.

Changes Made:

• Upgraded to Go 1.18 to ensure compatibility with the latest dependencies.
• Added Go module initialization to handle package dependencies correctly.

Impact:

• Resolves build issues and ensures the Dockerfile works with the latest Go ecosystem.
• Improves the build process and compatibility for the guestbook application.

This update addresses the build error and aligns the Dockerfile with modern Go practices, ensuring a smoother development and deployment experience.

Issue - guestbook go build issue #540

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[AtharvaNawathe]

/easycla