docker: clarify compose env setup

.env.example

@@ -0,0 +1,22 @@
+# Copy this file to .env before running Docker Compose:
+#   cp .env.example .env
+
+# Published image to run. Use a different repository or tag if you are testing
+# a fork or a specific release.
+PYMC_REPEATER_IMAGE=pymcdev/pymc-repeater:dev
+# PYMC_REPEATER_IMAGE=yellowcooln/pymc-repeater:dev
+
+# SPI/GPIO access uses the host's numeric group IDs. Check your host with:
+#   getent group gpio
+#   getent group spi
+# Example output:
+#   gpio:x:997:
+#   spi:x:999:
+# Put the third field from each output line below.
+GPIO_GID=986
+SPI_GID=989
+
+# Local build only. These are used by docker-compose.build.yml if you build the
+# image yourself instead of pulling PYMC_REPEATER_IMAGE.
+PUID=15888
+PGID=15888