Bless Earthly Docker Image

[dchw]

Revamp entrypoint to make it more user-friendly and button it down more. Dogfoods it within our own unit testing. Docs coming once merged so I don't have to keep those up to date too with any changes here.

Sample invocation 1:

docker run --privileged -t -e NO_DOCKER=1 -v $(pwd):/src earthly/earthly:v0.5.15 +for-linux

• -t lets Earthly detect that there is a TTY. Better than using FORCE_COLOR, when possible.
• -e NO_DOCKER=1 Skips the Docker check. Useful for when you don't need to save images, etc.
• -v $(pwd):/src is mounting the current dir into the conventional spot for invoking earthly.

Sample invocation 2:

docker run --privileged -t -v ~/.docker:/root/.docker -v /var/run/docker.sock:/var/run/docker.sock -e DOCKER_HOST="unix:///var/run/docker.sock" -v $(pwd):/src earthly/earthly:v0.5.15 +for-linux

• Note that we mount the Docker socket plus specify the DOCKER_HOST variable. If the daemon is remote or accessible otherwise, the socket mount would not be needed. Also note mounting .docker; this preserves any credentials. Should we support a docker login here?

[vladaionescu]

Mostly nits, hence approve