chore: harden container base images for SCA findings

[stevologic]

Motivation

• SCA tooling could not be run in this environment due to outbound restrictions to Go proxy/GitHub and missing scanner binaries, so base images were pinned/updated to reduce known CVE exposure, supply-chain drift, and improve reproducibility.

Description

• Updated backend build image from golang:1.24-alpine to golang:1.24.6-alpine3.21, backend runtime from alpine:3.18 to alpine:3.21, and UI image from nginx:alpine to nginx:1.27-alpine3.21 by modifying Dockerfile and ui/Dockerfile.

Testing

• Ran go test ./... which passed, and attempted govulncheck and go list -m -u (and other SCA scanner invocations) which failed due to network/Go proxy/GitHub restrictions and missing scanner binaries in the environment.

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Codex Task