docker: switch to distroless runtime

[cristi-]

Switch to gcr.io/distroless/cc-debian13 for smaller images. Ubuntu debug target available via --target debug.

Test plan

• Built distroless and debug targets
• Smoke tested both images

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

Switches the production Docker image from ubuntu:latest to gcr.io/distroless/cc-debian13 for smaller image sizes. Adds a separate debug target that retains Ubuntu with shell access for troubleshooting.

• Production image now uses distroless, which includes CA certificates and C runtime libraries needed by the Rust binary
• Debug target available via docker build --target debug preserves the original Ubuntu-based image with shell access
• Build stage unchanged, continues to use jemalloc feature

Confidence Score: 5/5

• This PR is safe to merge - straightforward Dockerfile change with no code logic modifications
• Simple infrastructure change that swaps base image. The distroless cc-debian13 image includes all necessary runtime dependencies (CA certificates, glibc, libgcc). Author confirmed smoke testing both targets.
• No files require special attention

Important Files Changed

Filename	Overview
Dockerfile	Switched from Ubuntu to distroless cc-debian13 for smaller production images, added Ubuntu debug target for troubleshooting. No issues found.

Sequence Diagram

sequenceDiagram
    participant User as User
    participant DockerBuild as Docker Build
    participant Builder as Builder Stage
    participant Runtime as Runtime Stage

    User->>DockerBuild: docker build .
    DockerBuild->>Builder: rust:latest
    Builder->>Builder: cargo build --release
    Builder->>Builder: Copy binary from cache
    
    alt default (runtime target)
        DockerBuild->>Runtime: gcr.io/distroless/cc-debian13
        Runtime->>Runtime: Copy /app/cachey
    else --target debug
        DockerBuild->>Runtime: ubuntu:latest
        Runtime->>Runtime: apt install ca-certificates
        Runtime->>Runtime: Copy /app/cachey
    end
    
    Runtime->>User: ENTRYPOINT ["./cachey"]

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