fix: use official hexpm/elixir images instead of erlang-solutions repo

The Erlang Solutions APT repository was experiencing 504 Gateway Timeout
errors during builds, making the image build process unreliable.

Changes:
- Use official hexpm/elixir Docker image as builder stage
- More reliable (official Docker Hub images)
- Faster builds (pre-built binaries, no package installation)
- Still produces minimal runtime-only final image
- Updated documentation to reflect new approach

Benefits:
- No dependency on erlang-solutions CDN availability
- Faster build times (no apt-get install of large packages)
- Same minimal final image size (~150-200 MB)
- Easier to specify exact Erlang/Elixir versions

k8s/base-image/Dockerfile

@@ -1,5 +1,5 @@
 # Minimal Debian 13 (Trixie) with Erlang/OTP and Elixir runtime
-# Built from scratch with only essential packages
+# Uses official hexpm/elixir images as builder for reliability
 #
 # This image is designed to be rebuilt regularly to get latest security updates
 
@@ -7,30 +7,10 @@ ARG DEBIAN_VERSION=trixie-20251229-slim
 ARG ERLANG_VERSION=28.3
 ARG ELIXIR_VERSION=1.19.5
 
-FROM docker.io/debian:${DEBIAN_VERSION} as builder
+# Use official Elixir image as builder (more reliable than erlang-solutions repo)
+FROM docker.io/hexpm/elixir:${ELIXIR_VERSION}-erlang-${ERLANG_VERSION}-debian-${DEBIAN_VERSION} as builder
 
-# Install build dependencies (will be removed in final image)
-RUN apt-get update \
-    && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
-       wget \
-       ca-certificates \
-       gnupg2 \
-    && rm -rf /var/lib/apt/lists/*
-
-# Add Erlang Solutions repository for latest Erlang/OTP
-RUN wget -O- https://packages.erlang-solutions.com/ubuntu/erlang_solutions.asc | \
-    gpg --dearmor -o /usr/share/keyrings/erlang-solutions.gpg \
-    && echo "deb [signed-by=/usr/share/keyrings/erlang-solutions.gpg] https://packages.erlang-solutions.com/debian trixie contrib" \
-       > /etc/apt/sources.list.d/erlang.list
-
-# Install Erlang/OTP and Elixir
-RUN apt-get update \
-    && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
-       esl-erlang \
-       elixir \
-    && rm -rf /var/lib/apt/lists/*
-
-# Verify installations
+# Verify installations in builder
 RUN erl -eval 'erlang:display(erlang:system_info(otp_release)), halt().' -noshell
 RUN elixir --version
 

k8s/base-image/README.md

@@ -42,8 +42,8 @@ chmod +x build.sh
 ```
 
 This builds a multi-stage Docker image with:
-- **Builder stage**: Installs Erlang and Elixir from Erlang Solutions repository
-- **Final stage**: Minimal Debian with only runtime dependencies
+- **Builder stage**: Uses official hexpm/elixir image with pre-built Erlang/Elixir
+- **Final stage**: Minimal Debian with only runtime dependencies copied from builder
 
 ### Updating System Packages
 
@@ -191,8 +191,8 @@ rebuild-base-image:
 ### Image fails to build
 
 ```bash
-# Check if Erlang Solutions repository is accessible
-wget -O- https://packages.erlang-solutions.com/ubuntu/erlang_solutions.asc
+# Check if hexpm images are accessible
+docker pull hexpm/elixir:1.19.5-erlang-28.3-debian-trixie-20251229-slim
 
 # Try building with verbose output
 docker build --progress=plain --no-cache -f Dockerfile .
@@ -237,6 +237,6 @@ We use **Debian** instead of Alpine because:
 
 ## See Also
 
-- [Erlang Solutions Packages](https://www.erlang-solutions.com/downloads/)
+- [hexpm/elixir Docker Images](https://hub.docker.com/r/hexpm/elixir)
 - [Elixir Installation Guide](https://elixir-lang.org/install.html)
 - [Docker Multi-Stage Builds](https://docs.docker.com/build/building/multi-stage/)