more sensible Dockerfile for portable image

[dongdigua]

currently make serve won't create a usable local image because bind-mounted pwd overwrites the installed bundles in docker build image

podman run -it -p 4000:4000 --rm -v $(pwd)/:/GTFOBins/ gtfobins
Could not find nokogiri-1.19.0-x86_64-linux-gnu, addressable-2.8.8, typhoeus-1.5.0, faraday-2.14.0, ethon-0.15.0, execjs-2.10.0, activesupport-8.1.2, json-2.18.0, ffi-1.17.3-x86_64-linux-gnu, bigdecimal-4.0.1, minitest-6.0.1, prism-1.8.0 in locally installed gems
Run `bundle install` to install missing gems.

caused by 7505534

Also, it don't make sense to cd into the repo's repository and bind-mount. It would be nice to run it with a single podman run --rm -p 4000:4000 gtfobins in ANY directory