Add poetry to PATH

[TisVictress]

This change will resolve #123

[robdimsdale]

This looks exactly like what I expected. and I can see that it works when I run the following:

pack build my-app --path ./integration/testdata/poetry
docker run -it my-app --entrypoint=launcher poetry --version
Poetry version 1.1.13

I think we can add an integration test here, under the existing test called: builds an oci image with poetry (dependency management only).

I hacked a test with the following:

			container, err = docker.Container.Run.
				WithTTY().
				WithEntrypoint("launcher").
				WithCommand("poetry --no-ansi --version"). // Use the no-ansi flag to disable color output - required for regex to match
				Execute(image.ID)
			Expect(err).NotTo(HaveOccurred())

			Eventually(func() string {
				cLogs, err := docker.Container.Logs.Execute(container.ID)
				Expect(err).NotTo(HaveOccurred())
				return cLogs.String()
			}).Should(MatchRegexp(`Poetry version \d+\.\d+\.\d+`))

But I haven't validated whether we can re-use the existing container - we might need to create a new container just to run this separate command.

[TisVictress]

@robdimsdale Thanks for the suggested changes! I've updated the PR.

[robdimsdale]

LGTM - thanks for submitting this @TisVictress !