Support overriding base image via build arg (#571)

* Support overriding base image via build arg

Add BASE_IMAGE build arg to allow building Canasta with a custom
CanastaBase image. This enables the Canasta CLI's --build-from flag
to build both CanastaBase and Canasta from local source repositories.

Default behavior is unchanged (uses ghcr.io/canastawiki/canasta-base:1.0.7).

* Fix CI workflow to extract BASE_IMAGE from ARG line

The Dockerfile now uses an ARG for the base image instead of
hardcoding it in the FROM line. Update the awk command to parse
the default value from the ARG line.

Author: cicalese

.github/workflows/docker-image.yml

@@ -46,7 +46,7 @@ jobs:
         id: extract_mw
         run: |
           set -euo pipefail
-          BASE_IMAGE=$(awk '/^FROM ghcr\.io\/canastawiki\/canasta-base/ {print $2}' Dockerfile)
+          BASE_IMAGE=$(awk -F'=' '/^ARG BASE_IMAGE=/ {print $2}' Dockerfile)
           docker pull "$BASE_IMAGE" >/dev/null
           MW_VERSION=$(docker inspect --format '{{ index .Config.Labels "wiki.canasta.mediawiki.version" }}' "$BASE_IMAGE")
           echo "MW_VERSION=$MW_VERSION"

Dockerfile

@@ -1,4 +1,5 @@
-FROM ghcr.io/canastawiki/canasta-base:1.0.7 AS base
+ARG BASE_IMAGE=ghcr.io/canastawiki/canasta-base:1.0.7
+FROM ${BASE_IMAGE} AS base
 
 LABEL maintainers=""
 LABEL org.opencontainers.image.source=https://github.com/CanastaWiki/Canasta