ci: remove custom runner image approach (use public Docker Hub images instead)

Custom runner images were never needed. Forgejo runners pull label images
(golang:1.26-bookworm, node:22-bookworm, rust:1-bookworm) from Docker Hub
at job time. Node.js/docker are installed in workflow steps.

Removes dead-end Dockerfiles and build workflow that couldn't push through
Cloudflare's 100MB payload limit.

k8s/infra/forgejo-runner-images/Dockerfile.golang

@@ -1,12 +0,0 @@
-FROM golang:1.26-bookworm
-
-# Install Node.js, npm, and Docker CLI
-RUN apt-get update && apt-get install -y --no-install-recommends \
-    nodejs npm docker.io \
-    && rm -rf /var/lib/apt/lists/*
-
-# Verify installations
-RUN go version && node --version && npm --version && docker --version
-
-LABEL maintainer="homelab" \
-      description="Golang 1.26 + Node.js + Docker CLI pre-installed"

k8s/infra/forgejo-runner-images/Dockerfile.node

@@ -1,12 +0,0 @@
-FROM node:22-bookworm
-
-# Node.js already installed, add Docker CLI
-RUN apt-get update && apt-get install -y --no-install-recommends \
-    docker.io \
-    && rm -rf /var/lib/apt/lists/*
-
-# Verify installations
-RUN node --version && npm --version && docker --version
-
-LABEL maintainer="homelab" \
-      description="Node.js 22 + Docker CLI pre-installed"

k8s/infra/forgejo-runner-images/Dockerfile.rust

@@ -1,12 +0,0 @@
-FROM rust:1-bookworm
-
-# Add Node.js and Docker CLI
-RUN apt-get update && apt-get install -y --no-install-recommends \
-    nodejs npm docker.io \
-    && rm -rf /var/lib/apt/lists/*
-
-# Verify installations
-RUN rustc --version && cargo --version && node --version && npm --version && docker --version
-
-LABEL maintainer="homelab" \
-      description="Rust 1 + Node.js + Docker CLI pre-installed"