Pip will install packages for Python 3.6 by default

[outime]

After Python 3.7 was introduced (#22), this base image now has both Python 3.6 and Python 3.7. Python 3.6 is present in the Ubuntu's base image and purging it will cause unexpected side effects.

When pip is invoked it'll install libraries for 3.6. When the user runs its app through python, it'll run under 3.7 since it's set by update-alternatives, meaning that the app won't be able to use the installed dependencies.

I can only see few alternatives:

• Create a global alias for pip which would forward the call to python3.7 -m pip (which is available after python3-pip is installed). From the user point of view, this would be rather invisible.
• Last resort, which is rolling back to 3.6.

Either way, I can implement this or other alternatives myself if needed, since I was the one who pushed the change to 3.7. So, let me know what you think.

Edit: A separate issue, but it's still using 3.6.x for image tagging as well.

[hjacobs]

Let's make Python 3.7 the default in the base image.