Fallback search if container ip is empty. Updated install command in readme

[bschulz87]

This PR adds a fallback search in containers network settings if the regular container ip is an empty string.

Without this patch i got an error when using the proxy connection:

Can't forward traffic to backend tcp/:3306: ssh: rejected: connect failed (Temporary failure in name resolution)
Docker version 19.03.5, build 633a0ea838 (Remote Host)

I also updated the readme to use go install instead of go get

go install, with or without a version suffix (as described above), is now the recommended way to build and install packages in module mode. go get should be used with the -d flag to adjust the current module's dependencies without building packages, and use of go get to build and install packages is deprecated. In a future release, the -d flag will always be enabled. (https://tip.golang.org/doc/go1.16#modules)

[bschulz87]

Also feel free to remove the install change if i made a mistake here. I'm not super familiar with the go ecosystem to be honest.

[relistan]

cc @danpilch