Docker: Add docs on local building (testing).

[jtagcat]

~~add some docs on hey you can use make (#195 was my first 'developer' usage of make)~~

~~For docker, I think podsync should be output to bin/podsync instead, it seems that somebody worked around it ..but as the docker binary has a set architecture and os, it might interfere with the normal build, I didn't make a PR for it for now.~~

~~If the PR is made, the docker build cmd should have: -f Dockerfile bin (as docker magic, files in bin are ignored, but setting bin as the root will work, but then the dockerfile is in repo root... also the existing dockerfile will not need any modifications)~~

~~I can self-assign, but can't promise a date of completion, see jtagcat/jtagcat~~

https://github.com/mxpv/podsync/pull/249 (merge reverted) pretty much

If you go build it locally, and copy bin/podsync inside the container, when ran in the container, it fails with standard_init_linux.go:228: exec user process caused: no such file or directory