Update Dockerfile (gravitational#6499)

remove unnecessary apt-get update and small rework

Co-authored-by: Andrew Lytvynov <[email protected]>

docker/Dockerfile

@@ -1,13 +1,14 @@
 # The base image (buildbox:latest) is built by running `make -C build.assets`
 # from the base repo directory $GOPATH/gravitational.com/teleport
 FROM quay.io/gravitational/teleport-buildbox:go1.15.5
-RUN apt-get update
+
 # DEBUG=1 is needed for the Web UI to be loaded from static assets instead
 # of the binary
 ENV DEBUG=1 GOPATH=/root/go PATH=$PATH:/root/go/src/github.com/gravitational/teleport/build:/root/go/bin
 
 # htop is useful for testing terminal resizing
-RUN apt-get update; apt-get install -y htop vim screen; \
+RUN apt-get update && \
+    apt-get install -y htop vim screen && \
     mkdir -p /root/go/src/github.com/gravitational/teleport
 
 # allows ansible and ssh testing