dockerimg: do not include tmp-sketch-dockerfile in image

Fixes #39
diff --git a/dockerimg/testdata/testcreatedockerfile_empty_repo.dockerfile b/dockerimg/testdata/testcreatedockerfile_empty_repo.dockerfile
index fc93635..55e9245 100644
--- a/dockerimg/testdata/testcreatedockerfile_empty_repo.dockerfile
+++ b/dockerimg/testdata/testcreatedockerfile_empty_repo.dockerfile
@@ -7,8 +7,9 @@
     git config --global user.name "$GIT_USER_NAME" && \
     git config --global http.postBuffer 524288000
 
-LABEL sketch_context="1dc5390aeffdde5c41adc1f9d349206d7cdf1bf607e7dd1f4fd7d6f6b8c3be12"
+LABEL sketch_context="852a43dfbf76c6272f41ade86ac1b4567acb77141edfec6c1df20b07a4758d1a"
 COPY . /app
+RUN rm -f /app/tmp-sketch-dockerfile
 
 WORKDIR /app
 RUN if [ -f go.mod ]; then go mod download; fi
@@ -16,7 +17,19 @@
 # Switch to lenient shell so we are more likely to get past failing extra_cmds.
 SHELL ["/bin/bash", "-uo", "pipefail", "-c"]
 
-RUN if [ -f requirements.txt ]; then pip3 install -r requirements.txt || true; fi
+# Install any Go tools that might be useful for development
+go install github.com/golangci/golangci-lint/cmd/golangci-lint@latest || true
+go install github.com/rakyll/gotest@latest || true
+
+# Install Python dependencies if needed (with error handling)
+if [ -f requirements.txt ]; then
+    pip3 install -r requirements.txt || true
+fi
+
+# If Makefile exists, run make prepare or similar setup target
+if [ -f Makefile ]; then
+    grep -q "prepare:" Makefile && make prepare || true
+fi
 
 # Switch back to strict shell after extra_cmds.
 SHELL ["/bin/bash", "-euxo", "pipefail", "-c"]