[BUG] - official docker image cannot pip install python playwright

[rsalmond]

Context:

• Playwright Version: [what Playwright version do you use?] python playwright 1.9.2
• Operating System: [e.g. Windows, Linux or Mac] Linux (by way of the official docker image)
• Node.js version: [e.g. 10.12, 10.14] v14.16.0 (also by way of the official docker image)
• Browser: [e.g. All, Chromium, Firefox, WebKit] n/a
• Extra: [any specific details about your environment]

Describe the bug

The official docker image contains python2, python3.6, and python3.8 and it uses update-alternatives to make pip3 the default pip and python3.6 the default python. However in this configuration pip3 (which invokes python3) starts up with python3.6, since there is no update-alternatives command specifying that python3 should be python3.8. Because of this, pip/pip3 will try to search for a copy of playwright to use with python3.6, and will fail to find anything.

$ docker run -it mcr.microsoft.com/playwright:bionic bash
root@1f4ff0677fee:/# pip3 install playwright
Collecting playwright
  Could not find a version that satisfies the requirement playwright (from versions: )
No matching distribution found for playwright

root@1f4ff0677fee:/# pip3 --version
pip 9.0.1 from /usr/lib/python3/dist-packages (python 3.6)      <================

root@1f4ff0677fee:/# update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 10
update-alternatives: using /usr/bin/python3.8 to provide /usr/bin/python3 (python3) in auto mode

root@1f4ff0677fee:/# pip3 --version
pip 9.0.1 from /usr/lib/python3/dist-packages (python 3.8)      <================

root@1f4ff0677fee:/# pip3 install playwright
Collecting playwright
  Downloading https://files.pythonhosted.org/packages/b7/fd/a755971645836850765149e212ccc6a9756494ea438ac0a9efec4f5f9002/playwright-1.9.2-py3-none-manylinux1_x86_64.whl (59.1MB)
    100% |████████████████████████████████| 59.1MB 30kB/s
Collecting pyee>=8.0.1 (from playwright)
  Downloading https://files.pythonhosted.org/packages/0d/0a/933b3931107e1da186963fd9bb9bceb9a613cff034cb0fb3b0c61003f357/pyee-8.1.0-py2.py3-none-any.whl
Collecting greenlet==1.0.0 (from playwright)
  Downloading https://files.pythonhosted.org/packages/6d/59/b8e9baf19c1aaa438c1a196a007a1cc78f55877d7b860ac24e203066e911/greenlet-1.0.0-cp38-cp38-manylinux1_x86_64.whl (165kB)
    100% |████████████████████████████████| 174kB 6.2MB/s
Collecting typing-extensions (from playwright)
  Downloading https://files.pythonhosted.org/packages/60/7a/e881b5abb54db0e6e671ab088d079c57ce54e8a01a3ca443f561ccadb37e/typing_extensions-3.7.4.3-py3-none-any.whl
Installing collected packages: pyee, greenlet, typing-extensions, playwright
Successfully installed greenlet-1.0.0 playwright-1.9.2 pyee-8.1.0 typing-extensions-3.7.4.3