dockerimg: restore sketch_context docker label

The sketch_context label was removed in commit 2a5bd6d (switch to debian base image)
but the code that reads this label in findOrBuildDockerImage was left intact.

This was causing Docker to always rebuild images since the label it was checking
for no longer existed in the Dockerfile template.

Restore the label and updates the template.Execute call to pass the required
InitFilesHash parameter.

Updates #43
diff --git a/dockerimg/testdata/testcreatedockerfile_empty_repo.dockerfile b/dockerimg/testdata/testcreatedockerfile_empty_repo.dockerfile
index fb2fc22..94010f1 100644
--- a/dockerimg/testdata/testcreatedockerfile_empty_repo.dockerfile
+++ b/dockerimg/testdata/testcreatedockerfile_empty_repo.dockerfile
@@ -6,15 +6,19 @@
 RUN git config --global user.email "$GIT_USER_EMAIL" && \
     git config --global user.name "$GIT_USER_NAME"
 
+LABEL sketch_context="8680db4c498365658a6e773fde0d43f642ae6744f41a14ad6d01e9eae7064f65"
 COPY . /app
 
 WORKDIR /app
 RUN if [ -f go.mod ]; then go mod download; fi
 
-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
+# Install any Python dependencies but continue on error
+RUN if [ -f requirements.txt ]; then pip3 install -r requirements.txt || true; fi
+
+# Setup Go environment properly
+RUN if [ -f go.sum ]; then go mod tidy || true; fi
+
+# Make sure build scripts are executable
+RUN find . -name "*.sh" -exec chmod +x {} \; || true
 
 CMD ["/bin/sketch"]