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_empty_repo.dockerfile b/dockerimg/testdata/testcreatedockerfile_empty_repo.dockerfile
index b53e8fa..fb2fc22 100644
--- a/dockerimg/testdata/testcreatedockerfile_empty_repo.dockerfile
+++ b/dockerimg/testdata/testcreatedockerfile_empty_repo.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
@@ -6,13 +6,15 @@
 RUN git config --global user.email "$GIT_USER_EMAIL" && \
     git config --global user.name "$GIT_USER_NAME"
 
-LABEL sketch_context="17320ef95f67844208ac1b3e00f1f4dd0951e229517619bdbc8085eba3a7e067"
 COPY . /app
 
 WORKDIR /app
 RUN if [ -f go.mod ]; then go mod download; fi
 
-RUN apk add --no-cache python3 py3-pip || true
-RUN pip3 install --upgrade pip || true
+RUN set -eux; \
+    # Install any Python dependencies safely
+    pip3 install --no-cache-dir setuptools wheel || true; \
+    # Make sure sketch is executable
+    if [ -f /bin/sketch ]; then chmod +x /bin/sketch; fi
 
 CMD ["/bin/sketch"]