feat: expose runtime markers

[mtojek]

Fixes: #212

This PR adds logic to set extra environment variables to indicate the runtime.

See DEVCONTAINER, ENVBUILDER, and DEVCONTAINER_CONFIG:

root ➜ /workspaces/deno (main) $ env
DEVCONTAINER=true
HOSTNAME=da273e13e9d3
PWD=/workspaces/deno
HOME=/root
CARGO_HOME=/usr/local/cargo
DENO_INSTALL=/usr/local
TERM=xterm
RUSTUP_HOME=/usr/local/rustup
USER=root
INIT_SCRIPT=bash
GIT_URL=https://github.com/denoland/deno
SHLVL=1
PROMPT_DIRTRIM=4
KANIKO_DIR=/.envbuilder
ENVBUILDER=true
RUST_VERSION=1.78.0
PATH=/usr/local/cargo/bin:/usr/local/cargo/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/root/.local/bin
DOCKER_CONFIG=/.envbuilder
DEVCONTAINER_CONFIG=/workspaces/deno/.devcontainer/devcontainer.json
_=/usr/bin/env

[dannykopping]

LGTM