Dockerfile: add ${GOPATH}/bin to $PATH

[korniltsev]

This allows using the installed tools, such as porto in the docker dev container

root@3f484fa80ce1:/agent/src# make lint
GOARCH=arm64 go generate ./...
(cd support && ./generate.sh)
go: downloading github.com/jcchavezs/porto v0.7.0
types_gen.go
go: downloading golang.org/x/mod v0.21.0
bash: line 1: porto: command not found
(run: make vanity-import-fix)
make: *** [Makefile:100: vanity-import-check] Error 1

[florianl]

When testing this change, I noticed, this modification of PATH only does have an effect for the creation of the docker image.
If you build and start this docker image, you will notice this by:

# echo $PATH
/usr/local/cargo/bin:/usr/local/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

We might want to update the following line instead:

# Append to /etc/profile for login shells
RUN echo 'export PATH="/usr/local/go/bin:$PATH"' >> /etc/profile

[korniltsev]

oh, yes, thanks for catching this.
Indeed, I wanted to have the variable in the container not during the build. I will update the change.

[korniltsev]

Thanks, applied 12d168b