Read Poetry version from poetry.lock

bin/compile

@@ -25,9 +25,18 @@ for env_file in $BUILDPACK_VARIABLES ; do
     [ -f "$ENV_DIR/$env_file" ] && export "$(basename "$env_file")=$(cat "$ENV_DIR/$env_file" 2>/dev/null)"
 done
 
+POETRY_VERSION="${POETRY_VERSION:-}"
 if [ -z "${POETRY_VERSION:-}" ] ; then
-    export POETRY_VERSION=1.5.1
-    log "No Poetry version specified in POETRY_VERSION config var. Defaulting to $POETRY_VERSION."
+    if [ -f poetry.lock ]; then
+        POETRY_VERSION=$(head --lines=1 poetry.lock | grep --only-matching '\d\+\.\d\+\.\d\+')
+    fi
+    if [ -z "$POETRY_VERSION" ] ; then
+        export POETRY_VERSION=1.5.1
+        log "No Poetry version specified in POETRY_VERSION config var. Defaulting to $POETRY_VERSION."
+    else
+        export POETRY_VERSION
+        log "Using Poetry version from poetry.lock: $POETRY_VERSION"
+    fi
 else
     log "Using Poetry version from POETRY_VERSION config var: $POETRY_VERSION"
 fi