Update docker file (#719)

Pin base image ubuntu LTS version. add installation python3 with python3-yaml package. create python link in PATH for build hackrf dependency,

dockerfile

@@ -1,6 +1,6 @@
 #Download base image.
-#The ubuntu:latest tag points to the "latest LTS"
+#The ubuntu:20.04 tag points to the "20.04 LTS"
-FROM ubuntu:latest
+FROM ubuntu:20.04
 
 #Set location to download ARM toolkit from.
 # This will need to be changed over time or replaced with a static link to the latest release.
@@ -14,7 +14,7 @@ COPY ./ /havocsrc
 
 #Fetch dependencies from APT
 RUN apt-get update && \
-	apt-get install -y tar wget dfu-util cmake python bzip2 curl && \
+        apt-get install -y tar wget dfu-util cmake python bzip2 curl python3 python3-yaml && \
         apt-get -qy autoremove
 
 #Install current pip from PyPa
@@ -23,7 +23,9 @@ RUN curl https://bootstrap.pypa.io/pip/3.4/get-pip.py -o get-pip.py && \
 
 #Fetch additional dependencies from Python 2.x pip
 RUN pip install pyyaml
-RUN ln -s /usr/bin/python3 /usr/bin/python && \
+RUN rm -rf /usr/bin/python && \
+    rm -rf /usr/bin/pip && \
+    ln -s /usr/bin/python3 /usr/bin/python && \
     ln -s /usr/bin/pip3 /usr/bin/pip
 
 ENV LANG C.UTF-8
@@ -43,3 +45,4 @@ CMD cd /havocsrc && \
     mkdir build && cd build && \ 
     cmake .. && make firmware && \
     cp /portapack-havoc/firmware/portapack-h1-havoc.bin /havocbin
+