cli: add --dry-run option to 'popper run'

[ivotron]

Add a --dry-run option that just prints what the workflow would look like without actually executing it

[akashrajr1]

@ivotron can you explain this issue? Do we have to print the order of the actions being executed in a workflow?
For this workflow do we print the order of execution of actions like this - test init, test ci, test reuse, end.

[ivotron]

@akashrajr1 For example, for this workflow, the output looks like this:

[install] docker pull node:11.6.0
[install] docker create node:11.6.0 npm install
[install] docker start ...
[lint] docker build -t action/jshint /tmp/gha-demo/./.github/actions/jshint
[lint] docker create action/jshint 
[lint] docker start .
[test] docker pull node:11.6.0
[test] docker create node:11.6.0 npm test
[test] docker start .
[branch-filter] docker build -t actions/bin /tmp/actions/actions/bin/./filter
[branch-filter] docker create actions/bin branch master
[branch-filter] docker start .
[deploy] docker build -t actions/bin /tmp/actions/actions/bin/./sh
[deploy] docker create actions/bin env
[deploy] docker start .

For this issue, we can print a DRYRUN: prefix for every line, something like this:

DRYRUN: [install] docker pull node:11.6.0
DRYRUN: [install] docker create node:11.6.0 npm install
DRYRUN: [install] docker start ...
DRYRUN: [lint] docker build -t action/jshint /tmp/gha-demo/./.github/actions/jshint
DRYRUN: [lint] docker create action/jshint 
DRYRUN: [lint] docker start .
DRYRUN: [test] docker pull node:11.6.0
DRYRUN: [test] docker create node:11.6.0 npm test
DRYRUN: [test] docker start .
DRYRUN: [branch-filter] docker build -t actions/bin /tmp/actions/actions/bin/./filter
DRYRUN: [branch-filter] docker create actions/bin branch master
DRYRUN: [branch-filter] docker start .
DRYRUN: [deploy] docker build -t actions/bin /tmp/actions/actions/bin/./sh
DRYRUN: [deploy] docker create actions/bin env
DRYRUN: [deploy] docker start .

this would let the user know that nothing got executed, just printed.