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_basic_repo_with_readme.dockerfile b/dockerimg/testdata/testcreatedockerfile_basic_repo_with_readme.dockerfile
index b23552a..8ea2cdb 100644
--- a/dockerimg/testdata/testcreatedockerfile_basic_repo_with_readme.dockerfile
+++ b/dockerimg/testdata/testcreatedockerfile_basic_repo_with_readme.dockerfile
@@ -4,14 +4,15 @@
 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="1904f4ae13566afc998b89a591dd37bac3bbe5ee0811b3c8c5d017baa0b7a924"
+LABEL sketch_context="f6830cad46a9b0d71acb50ff81972f4494fe1a9d90869544b692bc26dd43adfb"
 COPY . /app
 
 WORKDIR /app
 RUN if [ -f go.mod ]; then go mod download; fi
 
-RUN echo "Setting up test project environment" || true
+# No additional setup required for this simple Go test project
 
 CMD ["/bin/sketch"]