Apparently the ppa for PHP has difficulty installing and needs this for some reason.

Dockerfile

@@ -5,7 +5,7 @@ ARG GRAV_VERSION=1.5.8
 
 # Install dependencies
 RUN apt-get update && \
-    apt-get install -y --no-install-recommends software-properties-common && \
+    apt-get install -y --no-install-recommends language-pack-en-base software-properties-common && \
     add-apt-repository ppa:ondrej/php && \
     apt-get update && \
     apt-get install -y --no-install-recommends sudo wget unzip && \