Normalize Go download version in devcontainer build to handle GO_VERSION=1.21

[Copilot]

Devcontainer builds failed in the Go install layer when GO_VERSION was set to a major.minor value (for example, 1.21), because the Dockerfile attempted to fetch a non-existent tarball name. This change makes Go version resolution robust for both short and fully-qualified version inputs.

• Version normalization

  • Introduced GO_DOWNLOAD_VERSION in the Go install step.
  • Converts major.minor input to major.minor.0 (e.g., 1.21 → 1.21.0).
  • Leaves already fully-qualified versions unchanged.

• Download path hardening

  • Switched Go artifact URL usage to a single computed tarball variable.
  • Uses go.dev/dl with explicit tarball naming derived from normalized version.

• Fail-fast integrity guard

  • Added non-empty artifact check (test -s) before extraction to avoid opaque tar extraction failures on bad/empty downloads.

GO_DOWNLOAD_VERSION="${GO_VERSION}" \
&& case "${GO_DOWNLOAD_VERSION}" in *.*.*) ;; *.*) GO_DOWNLOAD_VERSION="${GO_DOWNLOAD_VERSION}.0" ;; esac \
&& GO_TARBALL="go${GO_DOWNLOAD_VERSION}.linux-amd64.tar.gz" \
&& curl -fsSLO "https://go.dev/dl/${GO_TARBALL}" \
&& test -s "${GO_TARBALL}" \
&& tar -C /usr/local -xzf "${GO_TARBALL}"