feat: add make target, installdocker, for custompios docker build compatibility

[mitant]

Addresses https://github.com/mainsail-crew/crowsnest/issues/141

normal install target and supporting shell scripts untouched new installdocker target tested locally and builds on windows using custompios instructions for docker-based build of RatOS (mainsail)

Note: extra file changed because your develop branch is behind master by 1 commit ?

[mitant]

@KwadFan @mryel00 for your consideration. does not touch existing make target so shouldn't break anything :) try this if you run into issues using docker / custompios instructions per the other PR. I also forgot to mention that I am building on windows specifically, though it shouldn't matter.

[mryel00]

This PR solves the same issue as #158

158 is a more elegant solution to fix the problem of #141

Therefore I close this PR. Still a big thanks for your consideration to contribute to crowsnest 😄

[mitant]

👍