Fix Makefile (#117)

* Add the `PROJECT` variable to the .env file configurations and make the Makefile completely hermetic by removing the environment reading option which I previously missed.

* Update README.md to show the new `.env` file default configurations.

Makefile

@@ -17,7 +17,7 @@ COMMAND = /bin/zsh
 # Specify `PROJECT` for the `make` command if this is the case.
 _PROJECT = "${SERVICE}-${USR}"
 # The `COMPOSE_PROJECT_NAME` variable must be lowercase.
-PROJECT ?= $(shell echo ${_PROJECT} | tr "[:upper:]" "[:lower:]")
+PROJECT = $(shell echo ${_PROJECT} | tr "[:upper:]" "[:lower:]")
 PROJECT_ROOT = /opt/project
 
 # Creates a `.env` file in PWD if it does not exist.
@@ -45,6 +45,7 @@ GID=${GID}\n$\
 UID=${UID}\n$\
 GRP=${GRP}\n$\
 USR=${USR}\n$\
+PROJECT=${PROJECT}\n$\
 SERVICE=${SERVICE}\n$\
 COMMAND=${COMMAND}\n$\
 HOST_NAME=${SERVICE}\n$\

README.md

@@ -151,8 +151,12 @@ GID=1000
 UID=1000
 GRP=GROUPNAME
 USR=USERNAME
+PROJECT=train-username          # `PROJECT` must be in lowercase.
+SERVICE=train
+COMMAND=/bin/zsh                # Command to execute on starting the container.
 HOST_NAME=train
 IMAGE_NAME=cresset:train-USERNAME
+PROJECT_ROOT=/opt/project
 
 # [[Optional]]: Fill in these configurations manually if the defaults do not suffice.