Ctx mgr to ensure destuction of container-pip-deps.txt

The file container-pip-dependencies.txt was being left a directory when
building the docker image. This meant that it was being packaged when it
wasn't supposed to.

To avoid this, we remove file with the help from a context manager.

The change is minimal and the biggest part of the diff are indentation
changes.

Fixes #739

install/common/build-image.py

@@ -31,8 +31,7 @@ def main():
     args = parser.parse_args()
 
     print("Exporting container pip dependencies")
-    export_container_pip_dependencies()
+    with ContainerPipDependencies():
-
         print("Pulling base image")
         subprocess.run(
             [
@@ -97,7 +96,10 @@ def main():
         f.write(image_id)
 
 
-def export_container_pip_dependencies():
+class ContainerPipDependencies:
+    """Generates PIP dependencies within container"""
+
+    def __enter__(self):
         try:
             container_requirements_txt = subprocess.check_output(
                 ["poetry", "export", "--only", "container"], universal_newlines=True
@@ -110,6 +112,10 @@ def export_container_pip_dependencies():
         with open(Path(BUILD_CONTEXT) / REQUIREMENTS_TXT, "w") as f:
             f.write(req_txt_pymupdfb_stripped)
 
+    def __exit__(self, exc_type, exc_value, exc_tb):
+        print("Leaving the context...")
+        os.remove(Path(BUILD_CONTEXT) / REQUIREMENTS_TXT)
+
 
 if __name__ == "__main__":
     sys.exit(main())