Refactor install.sh (#765)

* [wip] get assets based on gh api

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* put install.sh download_asset fn under test

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* put install.sh install_asset fn under test

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* use zip for darwin installs

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* fix install.sh negative test cases

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* allow errors to propagate in install.sh

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* remove exit on error from install.sh tests

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* add more docs around install.sh helpers

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* add integration tests for install.sh

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* add install.sh testing to pipeline

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* add install test cache to CI

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* make colors globally available

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* test download against github release

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* always test release-based install against latest release

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* use better install.sh test names

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

test/install/environments/Dockerfile-alpine-3.6

@@ -0,0 +1,2 @@
+FROM alpine:3.6
+RUN apk update && apk add python3 wget unzip make ca-certificates

test/install/environments/Dockerfile-ubuntu-20.04

@@ -0,0 +1,2 @@
+FROM ubuntu:20.04
+RUN apt update -y && apt install make python3 curl unzip -y