safer comparison of MODE env var

[nvdk]

fixes #22

As mentioned there in the current setup one would still have the sh parser trip over a use case like

export MODE="my mode"
./start.sh: line 4: [: too many arguments

[MikiDi]

When running with this change, I get the error /start.sh: line 3: MODE: unbound variable on startup (and the script exits) when the MODE variable isn't set (which is the default). From the set documentation, I gather that this is expected behavior when considering this line in the start script:

mu-python-template/start.sh

Line 2 in 48b36f6

set -eu

I propose to give MODE a default value of production in the template Dockerfile, in line with the js template
https://github.com/mu-semtech/mu-javascript-template/blob/a2c9ed7ecd5e96592fe099596c5883ae58dd366a/Dockerfile#L9
or ruby template
https://github.com/mu-semtech/mu-ruby-template/blob/a24e6ef2becdd715961e70438be95ed39413bb4d/Dockerfile#L7

[madnificent]

@MikiDi I agree with the proposal. Templates should show roughly the same behavior so this would be a positive change.