dockerimg: switch to a debian base image

Developing on a musl-based alpine image seems to suck.
A lot of python stuff doesn't work.
Even when told it's alpine, LLMs still want to apt-get install things.

Along with switching to debian, simplify everything.
We *always* use debian bookwork now. If there are python
things, the LLM needs to install them on top of the model.
This will make fallback mode easier: if a build fails, then we
drop ExtraCmds and try again. (Future work.)

While here: avoid using docker buildx in tests because
it seems to vary a lot version-to-version.
diff --git a/dockerimg/testdata/testcreatedockerfile_basic_repo_with_readme.dockerfile b/dockerimg/testdata/testcreatedockerfile_basic_repo_with_readme.dockerfile
index 81a2a23..9a9e3a3 100644
--- a/dockerimg/testdata/testcreatedockerfile_basic_repo_with_readme.dockerfile
+++ b/dockerimg/testdata/testcreatedockerfile_basic_repo_with_readme.dockerfile
@@ -1,4 +1,4 @@
-FROM ghcr.io/boldsoftware/sketch:v1
+FROM ghcr.io/boldsoftware/sketch:8ad6c62da599d2e478ef79d6ef563630
 
 ARG GIT_USER_EMAIL
 ARG GIT_USER_NAME
@@ -6,12 +6,17 @@
 RUN git config --global user.email "$GIT_USER_EMAIL" && \
     git config --global user.name "$GIT_USER_NAME"
 
-LABEL sketch_context="2cfc5f8464b9aac599944b68917667ab7dd772029852cec23324f6e6b144ba70"
 COPY . /app
 
 WORKDIR /app
 RUN if [ -f go.mod ]; then go mod download; fi
 
-RUN apk add --no-cache bash git make jq sqlite gcc musl-dev linux-headers npm nodejs go github-cli ripgrep fzf python3 curl vim grep || true
+RUN go install golang.org/x/tools/cmd/stringer@latest
+
+# Install any potentially useful development tools
+RUN apt-get install -y --no-install-recommends gcc build-essential make || true
+
+# Try to setup Python environment if needed
+RUN python3 -m pip install --upgrade pip || true
 
 CMD ["/bin/sketch"]