Optimize Dockerfile and docker-compose for development

[gotjoshua]

Priorities:

1. Minimize time it takes for changes to src to be visible in the running app
2. Break Dockerfile Commands into more cachable chunks
3. Define ARGs and ENVs just before they are needed (to preserve cached sub-containers when changes occur)
4. Minimize Build time

Referencing Issue in wekan/wekan #1560

[gotjoshua]

My strategy, for now, is to get this commit from @andresmanelli working on a new branch here in wekan/wekan-dev (as is) and then to optimize it further.

Perhaps this optimization will eventually be useful for the main wekan/wekan Dockerfile, but that remains to be seen.