Add Docker support

[Fmstrat]

• Added a Dockerfile for full build support including apktool, zipalign, and apksigner
• Added DOCKER.md for full documentation
• Included link to DOCKER.md in README.md

Once this PR is merged, I'd be happy to help set up a GitHub Workflow to automatically build a ghcr.io/apktool:latest on merges into the main branch, and update the documentation to reflect that users can just pull the image vs building it themselves.

[iBotPeaches]

Thanks - I'll accept this and merge it. Just some comments/suggestions first.

[Fmstrat]

I think I've covered all the suggested optimizations, let me know if there's anything else!

[iBotPeaches]

I tried to build this locally and it could never complete due to the license approval. I see you have code to approve this, so a bit lost why its not auto approving these license agreements.

Downloading https://fdroid.gitlab.io/android-sdk-transparency-log/checksums.json into /root/.cache/sdkmanager/checksums.json
6 of 6 SDK package licenses not accepted.
Review licenses that have not been accepted (y/N)? 
Traceback (most recent call last):
  File "/usr/bin/sdkmanager", line 33, in <module>
    sys.exit(load_entry_point('sdkmanager==0.6.4', 'console_scripts', 'sdkmanager')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/sdkmanager.py", line 1182, in main
    build_package_list(use_net=True)
  File "/usr/lib/python3/dist-packages/sdkmanager.py", line 859, in build_package_list
    download_file(checksums_url + '.asc')
  File "/usr/lib/python3/dist-packages/sdkmanager.py", line 592, in download_file
    raise RuntimeError('304 Not Modified: ' + url)
RuntimeError: 304 Not Modified: https://fdroid.gitlab.io/android-sdk-transparency-log/checksums.json.asc
Traceback (most recent call last):
  File "/usr/bin/sdkmanager", line 33, in <module>
    sys.exit(load_entry_point('sdkmanager==0.6.4', 'console_scripts', 'sdkmanager')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/sdkmanager.py", line 1180, in main
    method(args.packages)
  File "/usr/lib/python3/dist-packages/sdkmanager.py", line 999, in install
    url = packages[key]
          ~~~~~~~~^^^^^
KeyError: ('build-tools', '')
The command '/bin/sh -c apt-get update &&  

[Fmstrat]

That's.. odd. What OS and docker version? Maybe I can reproduce.

[iBotPeaches]

That's.. odd. What OS and docker version? Maybe I can reproduce.

• Docker version 20.10.25+dfsg1, build b82b9f3
• #1 SMP PREEMPT_DYNAMIC Debian 6.5.3-1kali1 (2023-09-19)

Just my Kali laptop

[iBotPeaches]

I upgraded my machine, rebooted and its all good now. If the CI pipeline starts failing we can investigate further.