Schedule docker rebuild to run daily + remove unneeded docker build trigger

[jasminabrar]

• adding in a line to run the docker build action every day at midnight
• don't need to trigger the docker build action if a new pcell is added because ebeam action is modified now to copy all of the content of the repo into klayout's version of siepic ebeam pdk in the docker container. This ensures that the tests are referencing the most up to date repo files.