Build csb-docproxy image with new builder image feature

[jameshochadel]

Changes proposed in this pull request:

• This change adds a new feature to the OCI build step for the internal pipeline: The ability to specify a builder image. Many docker images are built using multi-stage builds: First, a more fully-featured builder image is used to build an application; next, the build artifacts are copied to a smaller final base image.
• The motivating use case is building a Go application related to the Cloud Service Broker. A builder step with the Go toolchain builds the application, and a final step copies the binary to our standard base image.
• Further motivation for the builder image here: Create terraform to deploy documentation proxy product#3216 (comment)
• Related to Create terraform to deploy documentation proxy product#3216

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

Updates our standard container process. Only applies to internal pipelines.