Fix the Docker build's dependency version mismatch

CHANGELOG.md

@@ -1,5 +1,6 @@
 # September 2026
 
+- [fixed] **The Docker image builds again** — the Dockerfile set `GAMEND_CONTENT_APP_VERSION=1.0.0` for the whole build, and gamend's `mix.exs` takes that variable as its own package version, so gamend_core and gamend_web compiled as 1.0.0 and the `>= 1.0.1266` floor rejected them ("Unchecked dependencies ... got 1.0.0"). The image no longer sets it; set it at runtime (`.env`) to change the version the API spec reports.
 - [changed] **Elixir 1.20 required** — the root `mix.exs` and both plugins ask for `~> 1.20`, as gamend does; the Dockerfile and CI already built on 1.20 / OTP 29. A `.tool-versions` with gamend's pins (`elixir 1.20`, `erlang 29.0.2`) is added, so `asdf install` sets them up.
 - [added] **gamend's `.credo.exs`** — `credo --strict` runs gamend's checks instead of Credo's defaults, and covers `modules/plugins/*/lib/` too.
 - [added] **Static assets are served compressed** — `config/prod.exs` turns on `gzip_static` and `brotli_static`, and `assets.deploy` ends with `bin/compress-static` (gamend's), which writes the `.br` files when `brotli` is installed and skips them otherwise.

Dockerfile

@@ -26,8 +26,10 @@
 ARG GAMEND_CONTENT_PLUGINS_DIR=modules/plugins
 ENV GAMEND_CONTENT_PLUGINS_DIR=${GAMEND_CONTENT_PLUGINS_DIR}
 
-ARG GAMEND_CONTENT_APP_VERSION=1.0.0
-ENV GAMEND_CONTENT_APP_VERSION=${GAMEND_CONTENT_APP_VERSION}
+# No GAMEND_CONTENT_APP_VERSION here, as an ARG or an ENV: gamend's own mix.exs
+# reads it as the engine's package version, so exporting it while the deps
+# compile builds gamend_core/gamend_web as that version, and mix.exs's
+# @gamend_min floor then rejects them. Set it at runtime (.env) instead.
 
 # Fetch top-level deps first for better layer caching.
 COPY mix.exs mix.lock ./
@@ -59,7 +61,7 @@
 # On container start, (re)build every plugin under GAMEND_CONTENT_PLUGINS_DIR before
 # serving. This is incremental — a no-op when nothing changed — and picks up code
 # changes when a plugin's lib/ is bind-mounted for live editing (see docker-compose).
 CMD set -e; \
     for plugin in "${GAMEND_CONTENT_PLUGINS_DIR}"/*/; do \
       [ -f "${plugin}mix.exs" ] || continue; \
       echo "Rebuilding plugin ${plugin}"; \