diff --git a/docker/Dockerfile b/docker/Dockerfile
index 9e5ea3f..c02230f 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -1,26 +1,27 @@
 # Build container
 FROM debian:11-slim AS builder
-RUN apt-get -y update && apt-get -y upgrade && apt-get -y install gcc libpcap-dev libvdeplug-dev libpcre3-dev libedit-dev libsdl2-dev libpng-dev libsdl2-ttf-dev build-essential && apt-get clean
-ADD simh-3.9-0.tgz /
-WORKDIR /simh-3.9-0
-RUN make pdp11
+RUN apt-get -y update && apt-get -y upgrade && apt-get -y install --no-install-recommends build-essential ca-certificates curl gcc libpcap-dev libvdeplug-dev && apt-get clean
+ARG SIMH_VERSION=3.9-0
+RUN curl -L https://github.com/simh/simh/archive/refs/tags/v${SIMH_VERSION}.tar.gz|tar -xzC /
+WORKDIR /simh-${SIMH_VERSION}
+RUN make pdp11 && ln /simh-${SIMH_VERSION}/BIN/pdp11 /pdp11
 
 # Final container
 FROM debian:11-slim
 ENV DISK_FILENAME=rq0.dsk
-RUN apt-get -y update && apt-get -y upgrade && apt-get -y install curl ed gcc iproute2 iptables libpcap-dev libpcre3 net-tools telnet vdeplug
+RUN apt-get -y update && apt-get -y upgrade && apt-get -y install --no-install-recommends ca-certificates curl ed iproute2 iptables libpcap-dev net-tools telnet vdeplug
 
 # Add Tini to handle reaping processes
-ENV TINI_VERSION 0.19.0
+ARG TINI_VERSION=0.19.0
 ADD https://github.com/krallin/tini/releases/download/v${TINI_VERSION}/tini /tini
 RUN chmod +x /tini
 ENTRYPOINT ["/tini", "--"]
 
 # Add CoreDNS to forward DNS based on /etc/resolv.conf
-ENV COREDNS_VERSION 1.11.1
+ARG COREDNS_VERSION=1.11.1
 RUN curl -L https://github.com/coredns/coredns/releases/download/v${COREDNS_VERSION}/coredns_${COREDNS_VERSION}_linux_amd64.tgz|tar -xzC /usr/local/bin/
 
-COPY --from=builder /simh-3.9-0/BIN/pdp11 /usr/local/bin/pdp11
+COPY --from=builder /pdp11 /usr/local/bin/pdp11
 
 # Uses pattern matching to optionally copy rq0.dsk.gz if it exists
 COPY startup.sh Corefile boot.ini.template rq0.dsk.g[z] /
diff --git a/docker/build.sh b/docker/build.sh
deleted file mode 100755
index 493d9b7..0000000
--- a/docker/build.sh
+++ /dev/null
@@ -1,17 +0,0 @@
-#!/bin/sh
-
-usage () {
-	echo "usage: $0 tag"
-	exit 1
-}
-
-[ -n "$1" ] && tag="$1" || usage
-
-file="simh-3.9-0.tgz"
-url="https://codeload.github.com/simh/simh/tar.gz/refs/tags/v3.9-0"
-
-cd "$(dirname $0)"
-
-[ -f "$file" ] || wget "$url" -O "$file"
-
-docker build -t "$tag" .
diff --git a/docker/startup.sh b/docker/startup.sh
index 76b562e..54f541f 100755
--- a/docker/startup.sh
+++ b/docker/startup.sh
@@ -1,6 +1,8 @@
 #!/bin/sh
 set -e
 
+[ -e "/dev/net/tun" ] || { echo "/dev/net/tun not found. Is the container running in privileged mode?"; exit 1; }
+
 [ -n "$DISK_FILENAME" ] || DISK_FILENAME="rq0.dsk"
 
 echo -n "Checking for disk... "