Incorrect documentation for building Ballista on Linux when using docker-compose

[andygrove]

Describe the bug
The following pages have incorrect instructions for building and running with docker-compose:

The page at https://github.com/apache/arrow-ballista/blob/master/benchmarks/README.md has:

cargo build --release
docker-compose up --build

The page at https://github.com/apache/arrow-ballista/blob/master/docs/source/user-guide/flightsql.md has these specific instructions for Linux:

cargo build --release --all --features flight-sql
docker-compose up --build

Using the latter as an example, docker-compose fails to start the scheduler:

Step 9/14 : COPY ballista/ui/scheduler/build /var/www/html
COPY failed: file not found in build context or excluded by .dockerignore: stat ballista/ui/scheduler/build: file does not exist

To Reproduce
Clone the repo into a new directory and follow the instructions above

Expected behavior

Additional context

[avantgardnerio]

@andygrove @ianmcook we need to get this merged into apache's CI & the docs: https://github.com/spaceandtimelabs/arrow-ballista/blob/master/.circleci/config.yml#L51

[andygrove]

I think you meant to ping @iajoiner