dockerimg: set http.postBuffer for git inside container

We sometimes see failures with git push from the innie to the outie.
It appears to be when there is a big binary in the commit
(which is usually because of a different bug).
The internet suggests the HTTP POST limit in the git client
may be an issue (1MB in the git version in debian bookworm),
so let's bump it up.
diff --git a/dockerimg/testdata/testcreatedockerfile_python_misery.dockerfile b/dockerimg/testdata/testcreatedockerfile_python_misery.dockerfile
index c74aeb1..d7cd328 100644
--- a/dockerimg/testdata/testcreatedockerfile_python_misery.dockerfile
+++ b/dockerimg/testdata/testcreatedockerfile_python_misery.dockerfile
@@ -4,26 +4,22 @@
 ARG GIT_USER_NAME
 
 RUN git config --global user.email "$GIT_USER_EMAIL" && \
-    git config --global user.name "$GIT_USER_NAME"
+    git config --global user.name "$GIT_USER_NAME" && \
+    git config --global http.postBuffer 524288000
 
-LABEL sketch_context="00e8cee1c3621b40cc6b977430392a621b3d8dea06b12191c082f9bda155832b"
+LABEL sketch_context="2c865c897e88f0bc021007a21d2ed036f3918b5e8b9dbbd5708662980afb4ee6"
 COPY . /app
 
 WORKDIR /app
 RUN if [ -f go.mod ]; then go mod download; fi
 
-# Install Python 3.11 and set it up as the default python3
+# Install Python 3.11 and DVC as required by the project
 RUN apt-get update && \
     apt-get install -y --no-install-recommends python3.11 python3.11-venv python3-pip || true && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/*
 
-# Create symlink to make python3.11 the default python3
-RUN if [ -f /usr/bin/python3.11 ]; then \
-      update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.11 1; \
-    fi
-
-# Install dvc (Data Version Control) as mentioned in README
+# Install DVC
 RUN pip3 install dvc || true
 
 CMD ["/bin/sketch"]