Add flex to build image and VICE preflight check

VICE's configure.ac uses AC_PROG_LEX, which needs flex — missing from
the build image caused a late, confusing "Could not find either flex
or lex!" failure deep inside configure. Add it to the Dockerfile and
to build_vice.sh's own preflight tool check so a missing flex fails
fast with a clear error. Bump build-image/VERSION to 2.

build-image/Dockerfile

@@ -25,13 +25,14 @@ ENV DEBIAN_FRONTEND=noninteractive \
 # git/unzip/curl/jq: checkout, SDK downloads, and the release workflow's
 # calls to the Gitea API (create release, upload assets).
 # python3-venv: pebble-tool's `sdk install` creates a venv per SDK version.
-# dos2unix/autoconf/automake/pkg-config/xa65/build-essential/gettext: host
-# tools required by the companion app's build_vice.sh (see that file's own
-# preflight check) to cross-compile VICE via autotools before NDK clang
-# takes over for the actual target compilation.
+# dos2unix/autoconf/automake/pkg-config/xa65/build-essential/gettext/flex:
+# host tools required by the companion app's build_vice.sh (see that file's
+# own preflight check, plus flex for VICE's AC_PROG_LEX-based configure) to
+# cross-compile VICE via autotools before NDK clang takes over for the
+# actual target compilation.
 RUN apt-get update && apt-get install -y --no-install-recommends \
         curl ca-certificates git unzip tar xz-utils python3 python3-venv file jq \
-        dos2unix autoconf automake pkg-config xa65 build-essential gettext \
+        dos2unix autoconf automake pkg-config xa65 build-essential gettext flex \
     && rm -rf /var/lib/apt/lists/*
 
 # --- Android SDK: cmdline-tools, platform, build-tools, NDK ---

build-image/VERSION

@@ -1 +1 @@
-1
+2