Dockerfile fix

[dreglad]

Dockerfile base image had changed from Alpine to Ubun, breaking Docker image build

PR checklist

• [x] Read the contribution guidelines.
• [x] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Fixes in Dockerfile:

1. Updated base image

As described in #12158, the Docker build is broken since the base image was changed from Alpine to Ubuntu.

2. Added GEN_DIR as Build Argument

The WORKDIR was previously set by an environment variable GEN_DIR at build time. This setup doesn't make sense as it always resolves to the same default value, which makes the Dockerfile harder to read and prone to errors if anyone actually tries to customize this value Assuming there's a valid reason for allowing this to be dynamically set, I've added a build argument (ARG instruction) before ENV, so it can actually be used effectively.