Make docker build quicker

[damccorm]

Building the Python SDK harness container takes minutes on my machine.

./gradlew :sdks:python:container:buildAll

BUILD SUCCESSFUL in 9m 33s

Possible lead: "We spend mins pulling cmd/beamctl deps."

beam/sdks/python/container/build.gradle

Line 38 in 47feeaf

// TODO(herohde): get the pkg subdirectory only, if possible. We spend mins pulling cmd/beamctl deps.

Imported from Jira BEAM-8277. Original Jira may contain additional context.
Reported by: ibzib.