Dockerfile: fix LegacyKeyValueFormat warnings

Dockerfile

@@ -1,6 +1,6 @@
 FROM docker.io/python:3-alpine
 
-ENV IN_CONTAINER 1
+ENV IN_CONTAINER=1
 
 RUN apk add --no-cache git sudo
 
@@ -8,6 +8,6 @@ COPY . /hosts
 
 RUN pip install --no-cache-dir --upgrade -r /hosts/requirements.txt
 
-ENV PATH $PATH:/hosts
+ENV PATH=$PATH:/hosts
 
 WORKDIR /hosts