chore: add ubuntu 22.04 build container

Adds .containers/Containerfile.ubuntu2204-build defining a minimal
Ubuntu 22.04 image with the JUCE Linux build deps preinstalled, and
updates .gitignore to track only that Containerfile while ignoring
any local container scratch files under .containers/.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: tucktuckg00se

.containers/Containerfile.ubuntu2204-build

@@ -0,0 +1,23 @@
+FROM ubuntu:22.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+        build-essential \
+        ca-certificates \
+        cmake \
+        git \
+        libasound2-dev \
+        libcurl4-openssl-dev \
+        libfreetype-dev \
+        libwebkit2gtk-4.1-dev \
+        libx11-dev \
+        libxcursor-dev \
+        libxinerama-dev \
+        libxrandr-dev \
+        pkg-config \
+        zip \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /work

.gitignore

@@ -46,3 +46,6 @@ nul
 info/
 .docs/
 .codex
+.containers/*
+!.containers/
+!.containers/Containerfile.ubuntu2204-build