Fix build errors related to builder image

[jameshochadel]

Changes proposed in this pull request:

• The change in Build csb-docproxy image with new builder image feature #151 broke external pipelines due to missing input.
• The csb-dockerproxy was also broken due to the Dockerfile being located in a subfolder.
• This change also reduces rebuilds of images.

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

None.